feat(plugin): write-path trigger — offer prior art before code is rewritten

The milestone headline. Auto-inject fires on the operator's prompt; the
moment reuse is actually lost is later, when the agent decides mid-task to
write a helper. A PreToolUse hook on Write|Edit now fires there.

Channel: `additionalContext` with NO permissionDecision, so the note reaches
Claude beside the tool result and the write is never blocked — a recall aid
must not be able to stop the operator's work. Plain stdout would have been
invisible to the model, and deny/ask would have made a nudge into a gate.

Two arms, different in kind:
- BY PLACE — a snippet recorded at this path (or its directory) is prior art
  by definition, not resemblance, so it is neither scored nor thresholded.
  This is what #2083's reverse lookup was built to answer.
- BY MEANING — semantic search restricted to snippets (new `note_type` filter
  on semantic_search_notes) over the code about to be written.
Place ranks first; the top-k cap spans both arms.

Gates carried over from milestone 93 verbatim: threshold, margin, session
dedup, titles-never-bodies. Own `source='write_path'` in retrieval_logs so
precision is tunable separately — the docstring records that the place arm
is unlogged and hands that to #2085.

Its own on/off in Settings but the SAME threshold/top-k: one "how loud may
Scribe be" knob is easier to reason about than two that drift, and splitting
them later is then a data-backed change rather than a guess.

Details worth keeping: the hook sends a REPO-RELATIVE path because that is
how locations are recorded; the git remote resolves to a project and is never
used as the location `repo` filter (different namespaces, would silently
match nothing); the endpoint stays a GET because a read-scoped API key cannot
POST and every other hook depends on that.

plugin.json 0.1.17 -> 0.1.18. Refs #2082, milestone #232.

plugin/README.md

@@ -7,6 +7,10 @@ instance into a first-class Claude Code extension:
   rulebook (the `scribe` server).
 - **Session-start push channel** — a `SessionStart` hook injects your always-on
   rules + active-project context so Scribe surfaces *without being asked*.
+- **Prior-art recall on writes** — a `PreToolUse` hook on Write/Edit checks the
+  file about to be written against your recorded snippets (what's kept at that
+  path, and what resembles the code) and offers them before the helper is
+  rewritten. Titles only, never blocks the edit.
 - **Universal process-skills** — using-scribe, writing-plans,
   systematic-debugging, verification, brainstorming, reusing-code (record and
   recall reusable code as snippets). Replaces superpowers.
@@ -42,6 +46,11 @@ On install you'll be asked for:
 - `hooks/hooks.json` → SessionStart hook (`hooks/scribe_session_context.sh`),
   **fail-open**: if Scribe is unreachable it injects nothing and never blocks
   the session.
+- `hooks/hooks.json` → PreToolUse hook on `Write|Edit`
+  (`hooks/scribe_prior_art.sh`) → `GET /api/plugin/prior-art`. Returns
+  `additionalContext` with **no** permission decision, so it can inform the write
+  but never stop it; silent when nothing is recorded, which is most of the time.
+  Toggle in **Settings → Knowledge auto-inject**.
 - `skills/` → the universal process-skills, surfaced by description match.
 - `hooks/scribe_sync_processes.sh` (a 2nd SessionStart hook) + the `/scribe:sync`
   command → generate `~/.claude/skills/scribe-proc-*` stubs from your Scribe
@@ -52,5 +61,6 @@ On install you'll be asked for:
 
 - Set a `version` bump in `.claude-plugin/plugin.json` per release so clients
   pick up changes.
-- The session-start hook needs only a **read**-scoped key; the MCP tools need
-  **write** scope to create/update.
+- The session-start, auto-inject and prior-art hooks need only a **read**-scoped
+  key; the MCP tools need **write** scope to create/update. Every hook is a GET
+  for that reason — a read key cannot POST.