feat(booth): kept boards — a .forever sentinel and a standing link board

Agent sessions hand the operator URLs and they drown in terminal
scrollback. The Booth is the right home for them — it already has the one
property that decides adoption, which is that a session can publish with
mkdir and cp, no API key, no schema, no deploy — but everything in it dies
in 24h.

So: a booth containing `.forever` is never swept, and renders in its own
Kept lane at the top of the index. Opt-in per booth, so the ephemeral
default is untouched and nobody inherits a cleanup chore. `rm` the
sentinel and the board rejoins the sweep; the CLI verbs are sugar over
exactly that, which keeps the filesystem-is-the-state model honest.

The pin is deliberately NOT wired into is_expired(). That stays a pure age
question feeding the `expires_in` countdown; only sweep_once() honours the
sentinel. Keeping expiry arithmetic and reaper policy apart means they
cannot drift into each other.

Kept cards are visually separated per Australis: a 2px top edge in aurora
blue, the one accent border the system sanctions. They show "kept" instead
of a countdown, and they deliberately lose the one-click wipe button — a ×
next to the durable stuff is a footgun, so removing a kept board is a
two-step act.

`booth link <url> [description]` appends to the standing `links` board,
creating and keeping it on first use. Entries carry provenance (handle or
hostname, plus a timestamp) because a bare URL is unreadable three days
later. The append is one printf of one line to an O_APPEND fd — atomic
under PIPE_BUF on POSIX — which matters because many agents post to one
board and interleaved half-lines would be the obvious failure mode.

Seven tests cover the sentinel: detection, survival of a sweep that wipes
its neighbour, the deliberate is_expired/sweep_once split, the listing
flag, the sentinel not inflating item counts, and both lane-rendering
directions. Two of them originally asserted on the bare strings "Kept" and
"kept-grid", which passed for the wrong reason — those also appear in the
inlined stylesheet served on every page — so they now assert the full
class attribute. 55 pass.

Also corrects the Homepage card's description, which advertised a flat 24h
TTL that is no longer the whole story.

tests/test_booth.py

@@ -7,6 +7,7 @@ from fastapi.testclient import TestClient
 
 from booth.app import (
     FAVICON_LINK,
+    KEEP_MARKER,
     build_gallery,
     classify,
     create_app,
@@ -14,6 +15,8 @@ from booth.app import (
     generate_pickup_id,
     human_dur,
     is_expired,
+    is_kept,
+    list_booths,
     render_doc,
     safe_upload_name,
     sweep_once,
@@ -528,3 +531,122 @@ def test_view_single_image_no_nav(client):
     assert r.status_code == 200
     # no arrow anchors with a single image (the .vnav CSS rule is always present)
     assert 'class="vnav vprev"' not in r.text and 'class="vnav vnext"' not in r.text
+
+
+# ---- kept booths: the `.forever` sentinel -----------------------------------
+#
+# The Booth's whole contract is "wiped 24h after last activity". A kept booth is
+# the deliberate exception: an operator-facing board (agent-posted links, a
+# standing report) that must outlive the sweep and stay separated from the
+# ephemeral traffic so it does not get lost in it.
+
+
+def _stale(path, seconds=10_000):
+    """Age a booth and everything in it well past any test TTL."""
+    t = time.time() - seconds
+    for p in sorted(path.rglob("*"), reverse=True):
+        os.utime(p, (t, t))
+    os.utime(path, (t, t))
+
+
+def test_is_kept_detects_the_sentinel(tmp_path):
+    plain = tmp_path / "plain"
+    plain.mkdir()
+    kept = tmp_path / "kept"
+    _touch(kept / KEEP_MARKER)
+
+    assert not is_kept(plain)
+    assert is_kept(kept)
+
+
+def test_kept_booth_survives_the_sweep(tmp_path):
+    """The point of the whole feature: expiry does not apply to a kept booth."""
+    doomed = tmp_path / "doomed"
+    _touch(doomed / "a.png")
+    _stale(doomed)
+
+    kept = tmp_path / "links"
+    _touch(kept / "a.png")
+    _touch(kept / KEEP_MARKER)
+    _stale(kept)
+
+    wiped = sweep_once(tmp_path, ttl_seconds=3600)
+
+    assert wiped == ["doomed"]
+    assert not doomed.exists()
+    assert kept.exists(), "a booth carrying the sentinel must never be swept"
+
+
+def test_kept_booth_is_still_reported_expired_by_age(tmp_path):
+    """is_expired stays a pure age question; only the sweeper honours the pin.
+
+    Keeping these separate means `expires_in` arithmetic and the reaper policy
+    cannot drift into each other.
+    """
+    kept = tmp_path / "links"
+    _touch(kept / KEEP_MARKER)
+    _stale(kept)
+
+    assert is_expired(kept, ttl_seconds=3600)
+    assert sweep_once(tmp_path, ttl_seconds=3600) == []
+
+
+def test_list_booths_flags_kept(tmp_path):
+    _touch(tmp_path / "ephemeral" / "a.png")
+    _touch(tmp_path / "links" / "a.png")
+    _touch(tmp_path / "links" / KEEP_MARKER)
+
+    by_name = {b["name"]: b for b in list_booths(tmp_path, ttl_seconds=3600)}
+
+    assert by_name["ephemeral"]["kept"] is False
+    assert by_name["links"]["kept"] is True
+
+
+def test_sentinel_is_not_counted_as_an_item(tmp_path):
+    """It is a dotfile, so it must not inflate the item count or become a tile."""
+    _touch(tmp_path / "links" / "a.png")
+    _touch(tmp_path / "links" / KEEP_MARKER)
+
+    booth = next(b for b in list_booths(tmp_path, ttl_seconds=3600) if b["name"] == "links")
+
+    assert booth["count"] == 1
+
+
+def test_index_separates_kept_from_ephemeral(client):
+    c, data = client
+    _touch(data / "scratch" / "a.png")
+    _touch(data / "links" / "a.png")
+    _touch(data / "links" / KEEP_MARKER)
+
+    html = c.get("/").text
+
+    # Assert on the lane's markup, not on the word "Kept" — that string also
+    # appears in the stylesheet comment that is served on every page, so a bare
+    # substring check passes for the wrong reason.
+    assert 'class="grid kept-grid"' in html, "kept booths need their own lane"
+    assert 'class="card card-kept"' in html
+    # The kept lane is rendered before the ephemeral grid, so the operator sees
+    # durable boards first rather than hunting for them among the churn.
+    assert html.index("links") < html.index("scratch")
+
+
+def test_kept_booth_shows_kept_instead_of_a_countdown(client):
+    c, data = client
+    _touch(data / "links" / "a.png")
+    _touch(data / "links" / KEEP_MARKER)
+
+    html = c.get("/").text
+
+    assert "expires in" not in html, "a kept booth has no expiry to advertise"
+
+
+def test_index_without_kept_booths_omits_the_lane(client):
+    c, data = client
+    _touch(data / "scratch" / "a.png")
+
+    html = c.get("/").text
+
+    # The full attribute form, because the bare class names also appear in the
+    # inlined stylesheet that ships on every page.
+    assert 'class="grid kept-grid"' not in html, "the lane must not render when nothing is kept"
+    assert 'class="card card-kept"' not in html