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.

frontend/src/views/SettingsView.vue

@@ -22,6 +22,7 @@ const retentionSaved = ref(false);
 const kbInjectEnabled = ref(true);
 const kbInjectThreshold = ref("0.55");
 const kbInjectTopK = ref("3");
+const kbWritePathEnabled = ref(true);
 const savingKbInject = ref(false);
 const kbInjectSaved = ref(false);
 
@@ -76,6 +77,9 @@ async function saveKbInject() {
       kb_autoinject_enabled: kbInjectEnabled.value ? 'true' : 'false',
       kb_autoinject_threshold: String(t),
       kb_autoinject_top_k: String(k),
+      // Its own switch, but deliberately the same threshold/ceiling — see
+      // WRITEPATH_ENABLED_KEY in services/plugin_context.py.
+      kb_writepath_enabled: kbWritePathEnabled.value ? 'true' : 'false',
     });
     kbInjectSaved.value = true;
     setTimeout(() => (kbInjectSaved.value = false), 2000);
@@ -459,6 +463,7 @@ onMounted(async () => {
   if (allSettings.kb_autoinject_top_k !== undefined) {
     kbInjectTopK.value = allSettings.kb_autoinject_top_k;
   }
+  kbWritePathEnabled.value = allSettings.kb_writepath_enabled !== "false";
   if (allSettings.notify_task_reminders !== undefined) {
     notifyTaskReminders.value = allSettings.notify_task_reminders !== "false";
   }
@@ -1193,6 +1198,19 @@ function formatUserDate(iso: string): string {
           />
           <p class="field-hint">Ceiling on titles surfaced at once (1–10).</p>
         </div>
+        <div class="checkbox-field">
+          <label>
+            <input type="checkbox" v-model="kbWritePathEnabled" />
+            Also surface prior art when Claude writes code
+          </label>
+          <p class="field-hint">
+            Checks the file Claude is about to write or edit against your recorded
+            snippets — what's already kept at that path, and what resembles the code
+            being written — so a helper you already have is offered before it's
+            rewritten. Uses the same threshold and ceiling above, and never blocks the
+            edit. Off = prior art surfaces only on your own prompts.
+          </p>
+        </div>
         <div class="actions">
           <button class="btn-save" @click="saveKbInject" :disabled="savingKbInject">
             {{ savingKbInject ? 'Saving…' : 'Save' }}