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/skills/reusing-code/SKILL.md

@@ -32,6 +32,11 @@ through recall/auto-inject; this skill is the active reflex around that.
   `location` points at the reference implementation. Adapt, don't re-derive.
 - If auto-inject already surfaced a snippet title that looks relevant, that's
   your cue to `get_snippet` it rather than start from scratch.
+- **Prior art offered beside a write is not noise — read it.** When Scribe notes
+  that a snippet is already recorded for the file you just wrote or edited, open
+  it before you go any further. Either it's the helper you were about to
+  duplicate — reuse it and drop yours — or it isn't, and the record needs the new
+  location adding. Both are cheaper now than after the duplicate settles in.
 
 ## The moment you build something reusable — record it