feat(manifest): U5 — booths that say who posted them and why

The index card showed a name, an item count and a countdown, and nothing
the poster chose. An agent with something to show therefore had no way to
make the booth say "look at this" and posted a URL to the link board
instead — which is why 145 of that board's 210 rows (69%) ended up
pointing at booths that had already been swept. The board was absorbing a
job it was never shaped for. This is the shape.

Each booth carries `.booth.json` — {handle, title, why, created} — written
by the CLI from $ALTHING_HANDLE, and the provenance line renders on both
index lanes and on the booth page header.

WHAT IS WHERE

- booth/manifest.py, stdlib-only and importing nothing from booth.* either:
  scripts/booth imports it under the system python3 with no venv, and a
  cross-import between two stdlib-only modules is a second way for that
  invariant to break. It joins the shared test_stdlib_only list and keeps
  a stricter copy of its own.
- The read is lenient and cannot raise. list_booths touches every booth on
  every index load, so a manifest that cannot be parsed costs that booth's
  provenance and nothing else. That is the v0.2.2 lesson applied before the
  same mistake rather than after it.
- Absent and damaged render differently — `unannounced` and `unreadable`.
  Folding "cannot be read" into "never said" would hide the one case
  somebody has to go and fix.
- Re-announcing preserves `created`. A second `booth add` sharpening the
  why is not a second appearance of the booth.
- The write is atomic (invariant 5); the temp file is itself a dotfile, so
  no listing can see it mid-write.

THREE OPERATOR CALLS, 2026-09-22

Flags on the existing new/add verbs rather than a separate `announce` verb
(a second step is the step that gets forgotten, which is the rot's own
mechanism). Unannounced booths get a quiet marker rather than nothing — the
convention is only adoptable if the gap is visible. U5 adds provenance only
and does NOT add a second index ordering keyed on announcement time; that
is a different surface needing its own stated rule, parked for v1.1.

NO EXEMPTION LIST

A pickup booth and the standing link board are created by the service, so
they announce themselves with handle `booth`, which is true rather than
manufactured. One rule — a booth with no manifest is unannounced — instead
of a growing set of special cases.

ALSO

tests/test_booth.py's keep/release assertion was slicing the page on the
bare word `boothhead`, which has lived in the stylesheet far longer than
the assertion has; it was reading CSS and passing on luck, and went red the
first time a new rule landed above the old one. Same assertion, aimed at
the markup. A U5 test had the mirror-image bug: pytest derives tmp_path
from the test name and the index renders data_dir, so a test named
`test_an_unannounced_booth_says_so` put the needle in the haystack itself
and passed against a template that did not yet exist.

310 tests (304 before this unit's CLI half). Live service restarted, 26/26
booth pages verified 200, end-to-end smoke through the real CLI.

NOT TAGGED. The cold contract-review panel is still in flight and the
code-review and bug-hunt gates have not run. Tagging with a gate
outstanding is what made v0.2.0 premature.

tests/test_cli.py

@@ -119,3 +119,91 @@ def test_answer_on_a_note_id_says_no_such_pick(booth):
     r = run(data, "answer", "b", "note-1")
     assert r.returncode == NO_SUCH_PICK
     assert "no such pick" in r.stderr
+
+
+# ---- U5: self-announcing booths ---------------------------------------------
+
+
+def _manifest(booth_dir):
+    import sys
+    sys.path.insert(0, str(pathlib.Path(__file__).parent.parent))
+    from booth.manifest import read_manifest
+    return read_manifest(booth_dir)
+
+
+def test_new_announces_the_booth(tmp_path):
+    """`$ALTHING_HANDLE` is the whole provenance story: the session already has
+    it, so the booth can say who made it without anybody typing a name."""
+    env = {**os.environ, "ALTHING_HANDLE": "shutter-dev"}
+    r = subprocess.run([str(SCRIPT), "new", "r18-ab", "--why", "pick the winner"],
+                       capture_output=True, text=True, timeout=30,
+                       env={**env, "BOOTH_DATA_DIR": str(tmp_path),
+                            "BOOTH_URL": "http://booth.invalid"})
+    assert r.returncode == 0, r.stderr
+    m = _manifest(tmp_path / "r18-ab")
+    assert m.handle == "shutter-dev"
+    assert m.why == "pick the winner"
+
+
+def test_new_without_a_why_is_still_legal(tmp_path):
+    """The flags are optional and existing call sites keep working. A booth
+    that says only who made it is still a booth that said something."""
+    r = subprocess.run([str(SCRIPT), "new", "scratch"], capture_output=True,
+                       text=True, timeout=30,
+                       env={**os.environ, "ALTHING_HANDLE": "booth-dev",
+                            "BOOTH_DATA_DIR": str(tmp_path),
+                            "BOOTH_URL": "http://booth.invalid"})
+    assert r.returncode == 0, r.stderr
+    m = _manifest(tmp_path / "scratch")
+    assert m.handle == "booth-dev" and m.why == ""
+
+
+def test_add_announces_and_still_copies_the_files(tmp_path):
+    """`add` is the verb most sessions actually use — it creates the booth AND
+    fills it — so the why has to ride on it or it rides nowhere."""
+    src = tmp_path / "src"
+    src.mkdir()
+    (src / "a.txt").write_text("content")
+    r = subprocess.run([str(SCRIPT), "add", "r18-ab", str(src / "a.txt"),
+                        "--why", "second pass", "--title", "R18 A/B"],
+                       capture_output=True, text=True, timeout=30,
+                       env={**os.environ, "ALTHING_HANDLE": "booth-dev",
+                            "BOOTH_DATA_DIR": str(tmp_path),
+                            "BOOTH_URL": "http://booth.invalid"})
+    assert r.returncode == 0, r.stderr
+    assert (tmp_path / "r18-ab" / "a.txt").read_text() == "content"
+    m = _manifest(tmp_path / "r18-ab")
+    assert m.why == "second pass" and m.title == "R18 A/B"
+
+
+def test_add_re_announcing_keeps_the_original_created(tmp_path):
+    """The common shape: `new` opens the booth, `add` drops the second batch and
+    sharpens the why. The booth appeared once."""
+    env = {**os.environ, "ALTHING_HANDLE": "booth-dev",
+           "BOOTH_DATA_DIR": str(tmp_path), "BOOTH_URL": "http://booth.invalid"}
+    src = tmp_path / "a.txt"
+    src.write_text("x")
+    subprocess.run([str(SCRIPT), "new", "b", "--why", "first"], check=True,
+                   capture_output=True, timeout=30, env=env)
+    first = _manifest(tmp_path / "b").created
+    subprocess.run([str(SCRIPT), "add", "b", str(src), "--why", "sharper"],
+                   check=True, capture_output=True, timeout=30, env=env)
+
+    after = _manifest(tmp_path / "b")
+    assert after.created == first
+    assert after.why == "sharper"
+
+
+def test_the_link_board_announces_itself_as_the_booths_own(tmp_path):
+    """No exemption list. The standing board is made by the service and posted
+    to by seventeen handles, so no single agent owns it — `booth` is the
+    truthful answer, and it keeps the rule to one line."""
+    r = subprocess.run([str(SCRIPT), "link", "http://example.invalid", "a thing"],
+                       capture_output=True, text=True, timeout=30,
+                       env={**os.environ, "ALTHING_HANDLE": "booth-dev",
+                            "BOOTH_DATA_DIR": str(tmp_path),
+                            "BOOTH_URL": "http://booth.invalid"})
+    assert r.returncode == 0, r.stderr
+    m = _manifest(tmp_path / "links")
+    assert m is not None and m.handle == "booth"
+    assert m.why