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/ink/termio/osc.ts

@@ -481,16 +481,16 @@ export const CLEAR_TAB_STATUS = osc(
 )
 
 /**
- * Gate for emitting OSC 21337 (tab-status indicator). Ant-only while the
- * spec is unstable. Terminals that don't recognize it discard silently, so
- * emission is safe unconditionally — we don't gate on terminal detection
+ * Gate for emitting OSC 21337 (tab-status indicator). Currently disabled
+ * (spec is unstable). Terminals that don't recognize it discard silently,
+ * so emission is safe unconditionally — we don't gate on terminal detection
  * since support is expected across several terminals.
  *
  * Callers must wrap output with wrapForMultiplexer() so tmux/screen
  * DCS-passthrough carries the sequence to the outer terminal.
  */
 export function supportsTabStatus(): boolean {
-  return process.env.USER_TYPE === 'ant'
+  return false
 }
 
 /**