feat: open useful USER_TYPE-gated features to all users (#644)

* feat: open useful USER_TYPE-gated features to all users

Remove 13 process.env.USER_TYPE === 'ant' gates that restricted useful
features to Anthropic employees. These features work without Anthropic
infrastructure and are now available to all open-build users.

Features opened:
- Agent nesting (sub-agents can spawn sub-agents)
- Effort 'max' persistence in settings
- Plan mode interview phase (controlled by feature flags)
- Sandbox disabled commands (via ~/.claude/feature-flags.json)
- All tips visible to all users (plan mode, feedback, shift-tab)

Simplified:
- Fullscreen defaults to off (use /config to enable)
- Explore agent always uses haiku model
- Plan mode tool uses conservative prompt for all users

Continues the USER_TYPE cleanup from #637 (dead code) and builds
on #639 (local feature flags).

* fix: address Copilot review comments — remove residual dead code

1. bridgeConfig.ts: ungate bridge override functions — return env vars
   directly instead of hardcoded undefined
2. bridgeMain.ts + initReplBridge.ts: ungate sessionIngressUrl — read
   CLAUDE_BRIDGE_SESSION_INGRESS_URL without USER_TYPE check
3. tools.ts: remove dead ConfigTool/TungstenTool imports, narrow
   eslint-disable scope, stub REPLTool/SuggestBackgroundPRTool to null
4. readOnlyValidation.ts: remove orphaned ANT_ONLY_COMMAND_ALLOWLIST
   and unused GH_READ_ONLY_COMMANDS import
5. insights.ts: remove entire remote collection plumbing (types,
   functions, options, display logic)
6. osc.ts: hardcode supportsTabStatus() to false (internal-only feature)
7. state.ts: simplify addSlowOperation/getSlowOperations to no-ops,
   remove dead constants

* fix: address Copilot review on PR #644

1. settings/types.ts: allow 'max' effort level for all users in Zod
   schema — was still gated behind USER_TYPE=ant, causing 'max' to be
   silently dropped on settings reload
2. shouldUseSandbox.ts: defensively normalize disabledCommands from
   feature flag config with Array.isArray() guards

* fix: address second round of Copilot review on PR #644

1. shouldUseSandbox.ts: validate top-level shape of disabledCommands
   before accessing properties (handles null/primitive from feature flag)
2. fullscreen.ts: update JSDoc to reflect removal of USER_TYPE default
3. osc.ts: update JSDoc — "Ant-only" → "Currently disabled"

src/utils/fullscreen.ts

@@ -107,15 +107,15 @@ export function _resetTmuxControlModeProbeForTesting(): void {
 
 /**
  * Whether fullscreen (flicker-free) mode is enabled. Env var takes highest
- * precedence, then the `flickerFreeMode` config setting, then the internal-only
- * default. External users can enable via `/config` instead of setting the env.
+ * precedence, then the `flickerFreeMode` config setting, then defaults to off.
+ * Users can enable via `/config` instead of setting the env.
  *
  * Priority order:
  *   CLAUDE_CODE_NO_FLICKER=0  → always off
  *   CLAUDE_CODE_NO_FLICKER=1  → always on (overrides tmux -CC guard too)
  *   tmux -CC detected         → off (corrupts terminal state)
  *   config flickerFreeMode    → on/off per user preference
- *   USER_TYPE=ant             → on by default for internal users
+ *   default                   → off
  */
 export function isFullscreenEnvEnabled(): boolean {
   // Explicit env opt-out always wins.
@@ -137,7 +137,7 @@ export function isFullscreenEnvEnabled(): boolean {
   // `/config` without having to set an env var.
   const configValue = getGlobalConfig().flickerFreeMode
   if (configValue !== undefined) return configValue
-  return process.env.USER_TYPE === 'ant'
+  return false
 }
 
 /**