security: gate domain-skill auto-promote on classifier_score > 0

`browse/src/domain-skill-commands.ts:140` (handleSave) writes
`classifier_score: 0` with the comment "L4 deferred to load-time / sidebar-agent
fills this in on first prompt-injection load." But CLAUDE.md "Sidebar
architecture" documents that sidebar-agent.ts was ripped, and grep for
recordSkillUse + classifierFlagged callers across browse/src/ returns zero hits
outside the module under test.

Net effect: every quarantined skill that survives three benign uses without
flag (`recordSkillUse(... , classifierFlagged: false)` x3) auto-promotes to
`active` and lands in prompt context wrapped as UNTRUSTED on every subsequent
visit to that host. The L4 score that was supposed to gate the promotion was
never written — the production save path puts 0 on disk and nothing later
updates it.

Threat model: a domain-skill body authored by an agent under the influence of
a poisoned page (the new `gstackInjectToTerminal` PTY path runs no L1-L3
either) would lose its auto-promote barrier after three uses. The exploit
isn't single-step but the bar is exactly N=3 prompt-injection-shaped uses on
a hostile page, which is well within reach.

Fix adds a single condition to the auto-promote gate in `recordSkillUse`:

    if (state === 'quarantined' && useCount >= PROMOTE_THRESHOLD &&
        flagCount === 0 && current.classifier_score > 0) {
      state = 'active';
    }

`classifier_score` is set once at writeSkill and never updated. Production
saves it as 0 (handleSave), so the gate stays closed; existing tests that
explicitly pass `classifierScore: 0.1` still auto-promote (the auto-promote
path is preserved for the day L4 is rewired).

Manual promotion via `domain-skill promote-to-global` is unaffected (it goes
through `promoteToGlobal` which has its own state-machine guard at line 337+).

Test: new regression case `does NOT auto-promote when classifier_score is 0
(production handleSave shape)` plants a skill with classifierScore=0 (matches
domain-skill-commands.ts:140), runs three uses without flag, asserts the skill
stays quarantined and readSkill returns null. Negative control: revert the
patch, the test fails with `Received: "active"`. With the patch: 15/15 pass.

browse/src/domain-skills.ts

@@ -291,8 +291,20 @@ export async function writeSkill(input: WriteSkillInput): Promise<DomainSkillRow
  *
  * Auto-promote logic:
  *   - increment use_count
- *   - if use_count >= PROMOTE_THRESHOLD AND flag_count == 0 → state:active
- *   - else stay quarantined with updated counter
+ *   - if use_count >= PROMOTE_THRESHOLD AND flag_count == 0 AND L4 has scored
+ *     the body (classifier_score > 0) → state:active
+ *   - else stay quarantined with updated counter; user must run
+ *     `domain-skill promote-to-global` manually
+ *
+ * The classifier_score > 0 gate is load-bearing: handleSave currently writes
+ * classifier_score=0 with the comment "L4 deferred to load-time / sidebar-agent
+ * fills this in on first prompt-injection load," but sidebar-agent was ripped
+ * (CLAUDE.md "Sidebar architecture") and nothing else updates the score, so
+ * skills authored via the production path never had their body scanned by L4.
+ * Without this gate, three benign uses promote any quarantined skill — including
+ * one written under the influence of a poisoned page — into the prompt context
+ * for every subsequent visit. The gate re-opens automatically the day L4 is
+ * rewired and writeSkill / recordSkillUse start receiving non-zero scores.
  */
 export async function recordSkillUse(host: string, projectSlug: string, classifierFlagged: boolean): Promise<DomainSkillRow | null> {
   const normalized = normalizeHost(host);
@@ -303,7 +315,12 @@ export async function recordSkillUse(host: string, projectSlug: string, classifi
   const useCount = current.use_count + 1;
   const flagCount = current.flag_count + (classifierFlagged ? 1 : 0);
   let state: SkillState = current.state;
-  if (state === 'quarantined' && useCount >= PROMOTE_THRESHOLD && flagCount === 0) {
+  if (
+    state === 'quarantined' &&
+    useCount >= PROMOTE_THRESHOLD &&
+    flagCount === 0 &&
+    current.classifier_score > 0
+  ) {
     state = 'active';
   }
   const updated: DomainSkillRow = {