feat: local feature flag overrides via ~/.claude/feature-flags.json (#639)

* feat: local feature flag overrides via ~/.claude/feature-flags.json

Replace the GrowthBook no-op stub with a local JSON file reader that
gives open-build users control over ~50 tengu_* feature flags without
needing Anthropic's GrowthBook server.

How it works:
- On first flag lookup, lazily reads ~/.claude/feature-flags.json
- Returns the configured value if the key exists, defaultValue otherwise
- When the file is absent, behavior is identical to the current stub
- CLAUDE_FEATURE_FLAGS_FILE env var overrides the file path (CI/testing)

Example ~/.claude/feature-flags.json:
  { "tengu_kairos_cron": true, "tengu_scratch": true }

Continues the infrastructure work from #315 and #352. This is a
prerequisite for replacing remaining USER_TYPE gates with local config.

* fix: use ESM imports and validate JSON shape in growthbook stub

- Replace require('fs'/'path'/'os') with ESM imports (node: prefix)
  to avoid ReferenceError in ESM bundle output
- Validate JSON.parse result is a plain object before using `in` operator
  to prevent TypeError on non-object JSON values

Addresses Copilot review comments on #639

* fix: reset flags cache in resetGrowthBook and refreshGrowthBookFeatures

Set _flags back to undefined so subsequent lookups re-read the JSON
file. Enables runtime reload and proper test isolation.

Addresses Copilot review comment on #639

* docs: explain why checkSecurityRestrictionGate is excluded from local flags

This is a remote killswitch for bypassPermissions mode — exposing it
via the local JSON file would let users accidentally disable
--dangerously-skip-permissions without understanding why.

* test: add unit tests for growthbook stub local feature flags

Covers: valid JSON loading, missing file fallback, malformed JSON,
non-object JSON (primitive, array), cache invalidation via
resetGrowthBook/refreshGrowthBookFeatures, all getter variants,
and checkSecurityRestrictionGate always returning false.

12 tests, 21 assertions.

* fix: use Object.hasOwn instead of in operator for flag lookup

Prevents inherited prototype properties (toString, constructor, etc.)
from being returned as flag values.

Addresses Copilot review comment on #639

* fix: align gate stub signatures and add Boolean coercion

Address remaining Copilot review feedback:
- checkSecurityRestrictionGate: accept gate param to match real signature
- checkStatsigFeatureGate/checkGate: coerce with Boolean() like real impl

scripts/no-telemetry-plugin.ts

@@ -34,28 +34,55 @@ export function _resetForTesting() {}
 `,
 
 	'services/analytics/growthbook': `
+import _fs from 'node:fs';
+import _path from 'node:path';
+import _os from 'node:os';
+
+let _flags = undefined;
+
+function _loadFlags() {
+  if (_flags !== undefined) return;
+  try {
+    const flagsPath = process.env.CLAUDE_FEATURE_FLAGS_FILE
+      || _path.join(_os.homedir(), '.claude', 'feature-flags.json');
+    const parsed = JSON.parse(_fs.readFileSync(flagsPath, 'utf-8'));
+    _flags = (parsed && typeof parsed === 'object' && !Array.isArray(parsed)) ? parsed : null;
+  } catch {
+    _flags = null;
+  }
+}
+
+function _getFlagValue(key, defaultValue) {
+  _loadFlags();
+  if (_flags != null && Object.hasOwn(_flags, key)) return _flags[key];
+  return defaultValue;
+}
+
 const noop = () => {};
 export function onGrowthBookRefresh() { return noop; }
 export function hasGrowthBookEnvOverride() { return false; }
-export function getAllGrowthBookFeatures() { return {}; }
+export function getAllGrowthBookFeatures() { _loadFlags(); return _flags || {}; }
 export function getGrowthBookConfigOverrides() { return {}; }
 export function setGrowthBookConfigOverride() {}
 export function clearGrowthBookConfigOverrides() {}
 export function getApiBaseUrlHost() { return undefined; }
 export const initializeGrowthBook = async () => null;
-export async function getFeatureValue_DEPRECATED(feature, defaultValue) { return defaultValue; }
-export function getFeatureValue_CACHED_MAY_BE_STALE(feature, defaultValue) { return defaultValue; }
-export function getFeatureValue_CACHED_WITH_REFRESH(feature, defaultValue) { return defaultValue; }
-export function checkStatsigFeatureGate_CACHED_MAY_BE_STALE() { return false; }
-export async function checkSecurityRestrictionGate() { return false; }
-export async function checkGate_CACHED_OR_BLOCKING() { return false; }
+export async function getFeatureValue_DEPRECATED(feature, defaultValue) { return _getFlagValue(feature, defaultValue); }
+export function getFeatureValue_CACHED_MAY_BE_STALE(feature, defaultValue) { return _getFlagValue(feature, defaultValue); }
+export function getFeatureValue_CACHED_WITH_REFRESH(feature, defaultValue) { return _getFlagValue(feature, defaultValue); }
+export function checkStatsigFeatureGate_CACHED_MAY_BE_STALE(gate) { return Boolean(_getFlagValue(gate, false)); }
+// Security killswitch — always false in the open build. Anthropic uses this
+// gate to remotely disable bypassPermissions mode; exposing it via local flags
+// would let users accidentally lock themselves out of --dangerously-skip-permissions.
+export async function checkSecurityRestrictionGate(gate) { return false; }
+export async function checkGate_CACHED_OR_BLOCKING(gate) { return Boolean(_getFlagValue(gate, false)); }
 export function refreshGrowthBookAfterAuthChange() {}
-export function resetGrowthBook() {}
-export async function refreshGrowthBookFeatures() {}
+export function resetGrowthBook() { _flags = undefined; }
+export async function refreshGrowthBookFeatures() { _flags = undefined; }
 export function setupPeriodicGrowthBookRefresh() {}
 export function stopPeriodicGrowthBookRefresh() {}
-export async function getDynamicConfig_BLOCKS_ON_INIT(configName, defaultValue) { return defaultValue; }
-export function getDynamicConfig_CACHED_MAY_BE_STALE(configName, defaultValue) { return defaultValue; }
+export async function getDynamicConfig_BLOCKS_ON_INIT(configName, defaultValue) { return _getFlagValue(configName, defaultValue); }
+export function getDynamicConfig_CACHED_MAY_BE_STALE(configName, defaultValue) { return _getFlagValue(configName, defaultValue); }
 `,
 
 	'services/analytics/sink': `