feat(booth): per-row link removal + render the link board as real UI

The standing link board is the one MULTI-WRITER booth -- every agent session
appends operator-facing URLs to it. "Delete the folder" was the only
granularity available, so removing one dead link meant hand-editing markdown.
It is 32 rows and only grows.

  booth links                 row number, entry id, raw row
  booth unlink 3              by row number
  booth unlink 8b40e0a5       by entry id (what the UI's x posts)
  POST /b/<name>/unlink       form field `entry` = content id

ROWS ARE ADDRESSED BY CONTENT ID, NEVER BY POSITION. The board is append-only
and multi-writer: another session can post between listing it and clicking x,
and an index would then delete a neighbour. An id either matches the row you
saw or matches nothing. A row number typed at the CLI is resolved to its id
BEFORE anything is deleted. Appends and prunes now take the same flock on
.links.lock, so a post cannot be lost inside a prune's read-modify-write.

UI: a booth carrying links.md renders as rows -- description, URL, provenance,
copy button, per-row x -- instead of a markdown blob. links.md is filtered out
of the gallery so it does not appear twice; the header counts LINKS not files;
the empty-state and the one-click "Wipe now" both stand down for a board (same
rule as the kept lane: nothing durable is one click from gone).

booth/links.py extracted, STDLIB ONLY. The CLI needs this logic and must not
require the service venv -- importing app.py drags in FastAPI, so deleting a
line from a text file would have needed a web framework installed.

THREE BUGS FOUND BY TESTING, all in the shell wrapper while the module was
correct throughout -- module-only tests would have caught none of them:

- `[ "$n" -eq 0 ] && echo ...` as the LAST statement made `booth links` exit 1
  whenever the board had rows. `unlink`'s index lookup calls it inside $( )
  under `set -e`, so a successful listing killed the caller and the removal
  silently did nothing while reporting success.
- ids are 8 hex chars and roughly one in forty is ALL DIGITS; those were read
  as row numbers, resolved to nothing, and removed nothing. Now disambiguated
  by the id's actual shape, not by "is it numeric".
- filtering links.md out of the gallery left `items` empty, so a full board
  rendered "This booth is empty" and an empty <div class="gallery"> under 32
  visible rows.

87 tests (was 76): parser tolerance of hand-written prose, content-id
stability across concurrent appends, removal precision, UI branch behaviour
for board/normal/empty booths, and subprocess CLI tests pinning the two shell
bugs. Deployed to nh3-dev and verified against the live 32-row board
read-only; board file byte-identical afterwards.

booth/app.py

@@ -23,6 +23,8 @@ State is the filesystem — `ls ~/booth-data` tells you everything. That is the
 from __future__ import annotations
 
 import asyncio
+import fcntl
+import hashlib
 import io
 import os
 import re
@@ -34,7 +36,7 @@ from contextlib import asynccontextmanager
 from pathlib import Path
 from urllib.parse import quote
 
-from fastapi import FastAPI, File, HTTPException, Request, UploadFile
+from fastapi import FastAPI, File, Form, HTTPException, Request, UploadFile
 from fastapi.responses import (
     FileResponse,
     HTMLResponse,
@@ -70,6 +72,17 @@ DOC_MAX_BYTES = 2 * 1024 * 1024  # above this, a doc is handed back raw, not ren
 # state anywhere but the filesystem.
 KEEP_MARKER = ".forever"
 
+# The link-board logic lives in booth/links.py (stdlib only) so the `booth` CLI
+# can use it without pulling FastAPI in. Re-exported here because call sites and
+# tests already reference these names through app.
+from booth.links import (  # noqa: E402
+    LINK_LOCK,
+    LINKS_FILE,
+    link_entry_id,
+    parse_link_entries,
+    remove_link_entry,
+)
+
 
 def doc_kind(name: str) -> str | None:
     """'markdown' | 'text' | None — a booth file viewable as a readable page."""
@@ -583,7 +596,21 @@ def create_app(
                 **base_ctx,
                 "name": name,
                 "name_url": quote(name, safe=""),
-                "items": build_gallery(booth),
+                # links.md is rendered AS the board below, so it must not also
+                # appear as a markdown doc tile — that would show the same
+                # content twice, once interactive and once not.
+                "items": [
+                    it for it in build_gallery(booth)
+                    if not ((booth / LINKS_FILE).is_file() and it["name"] == LINKS_FILE)
+                ],
+                # A booth carrying links.md is the standing link board: render
+                # its rows as real UI (link, provenance, per-row remove) instead
+                # of a markdown blob you can only edit by hand. Empty list for
+                # every other booth, so the template branch simply does not fire.
+                "board": (
+                    parse_link_entries((booth / LINKS_FILE).read_text())
+                    if (booth / LINKS_FILE).is_file() else []
+                ),
                 "uploaded": (booth / UPLOAD_MARKER).exists(),
                 "expires_in": max(0.0, ttl_seconds - booth_age_seconds(booth)),
             },
@@ -693,6 +720,22 @@ def create_app(
     # is what deletes. Anything relying on release-then-sweep is relying on a
     # 24h delay it probably did not intend.
 
+    @app.post("/b/{name}/unlink")
+    def board_unlink(name: str, entry: str = Form(...)):
+        """Remove ONE row from a link board, by content id.
+
+        Deliberately not by index: the board is append-only and multi-writer,
+        so between rendering the page and clicking × another session may have
+        posted. A content id either matches the row the operator saw or matches
+        nothing — it can never resolve to a neighbour.
+        """
+        removed = remove_link_entry(resolve_booth(name), entry)
+        if removed is None:
+            # Already gone (double-click, stale tab, someone else pruned it).
+            # Not an error worth a 404 page — the desired end state holds.
+            pass
+        return RedirectResponse(url=f"/b/{quote(name, safe='')}/", status_code=303)
+
     @app.post("/b/{name}/keep")
     def booth_keep(name: str):
         (resolve_booth(name) / KEEP_MARKER).touch()