feat: enable 15 additional feature flags in open build (#667)

* feat: enable 16 additional feature flags in open build

Activate features whose source is fully available in the mirror and
that have no Anthropic-internal infrastructure dependencies:

UI/UX: MESSAGE_ACTIONS, HISTORY_PICKER, QUICK_SEARCH, HOOK_PROMPTS
Reasoning: ULTRATHINK, TOKEN_BUDGET, SHOT_STATS
Agents: FORK_SUBAGENT, VERIFICATION_AGENT, MCP_SKILLS
Memory: EXTRACT_MEMORIES, AWAY_SUMMARY
Optimization: CACHED_MICROCOMPACT, PROMPT_CACHE_BREAK_DETECTION
Safety: TRANSCRIPT_CLASSIFIER
Debug: DUMP_SYSTEM_PROMPT

Also reorganize featureFlags into documented sections (disabled/upstream/new)
with inline comments explaining each flag's purpose.

* feat: add centralized GrowthBook defaults map for open build

Add _openBuildDefaults in the GrowthBook stub (no-telemetry-plugin.ts)
with all 66 runtime feature keys, organized by category with inline
comments describing each flag's purpose.

Override tengu_sedge_lantern (AWAY_SUMMARY) and tengu_hive_evidence
(VERIFICATION_AGENT) to true so these features work out of the box
without requiring manual ~/.claude/feature-flags.json setup.

Priority: feature-flags.json > _openBuildDefaults > upstream default

* feat: replace refusal language with positive security guidance

Remove refusal instructions from CYBER_RISK_INSTRUCTION since they are
redundant for Anthropic models (applied server-side) and useless for
uncensored models in multi-provider setups. Keep positive guidance for
security testing contexts and add red teaming support.

* Revert "feat: replace refusal language with positive security guidance"

This reverts commit 0463676a8f.

* fix: add EXTRACT_MEMORIES runtime gate overrides to open-build defaults

EXTRACT_MEMORIES was enabled at build-time but its runtime GrowthBook
gates (tengu_passport_quail, tengu_coral_fern) still defaulted to false,
preventing the feature from activating. Add both keys to
_openBuildDefaults so memory extraction works out of the box.

Also adds test coverage for _openBuildDefaults precedence behavior.

* docs: update GrowthBook runtime keys catalog to 88 keys

Expand the reference catalog in no-telemetry-plugin.ts from ~62 to 88
unique keys, covering all tengu_* call sites found in src/. Adds 27
previously undocumented keys including VSCode gates, dynamic configs
(auto-mode, cron, bridge), security gates, and KAIROS cron keys.

Adds "not exhaustive" disclaimer as suggested by Copilot reviewer.
Reorganizes categories with section dividers for readability.

scripts/no-telemetry-growthbook-stub.test.ts

@@ -50,6 +50,23 @@ describe('growthbook stub — local feature flag overrides', () => {
     expect(stub.getAllGrowthBookFeatures()).toEqual({})
   })
 
+  // ── Open-build defaults (_openBuildDefaults) ────────────────────
+
+  test('returns open-build default when flags file is absent', () => {
+    // tengu_passport_quail is in _openBuildDefaults as true; without a
+    // flags file the stub should return the open-build override, not
+    // the call-site defaultValue.
+    expect(stub.getFeatureValue_CACHED_MAY_BE_STALE('tengu_passport_quail', false)).toBe(true)
+    expect(stub.getFeatureValue_CACHED_MAY_BE_STALE('tengu_coral_fern', false)).toBe(true)
+  })
+
+  test('flags file overrides open-build defaults', () => {
+    // User-provided feature-flags.json takes priority over _openBuildDefaults.
+    writeFileSync(flagsFile, JSON.stringify({ tengu_passport_quail: false }))
+
+    expect(stub.getFeatureValue_CACHED_MAY_BE_STALE('tengu_passport_quail', true)).toBe(false)
+  })
+
   // ── Valid JSON object ────────────────────────────────────────────
 
   test('loads and returns values from a valid JSON file', () => {