fix(provider): add Use Anthropic option to switch back from third-party profiles

[0xghost42]

Fixes #1426

Problem

Once any third-party provider profile is active, /provider offers no way back to built-in Anthropic. getActiveProviderProfile falls back to profiles[0] whenever activeProviderProfileId is unset, so clearing the active id still re-selects a saved profile. The only escape was hand-editing ~/.openclaude.json (null the active id, empty providerProfiles) and a full restart — and auth status kept reporting third_party until then due to stale session env.

Fix

• Add an explicit ANTHROPIC_DEFAULT_PROFILE_ID sentinel. getActiveProviderProfile resolves it to undefined (built-in Anthropic) instead of falling through to profiles[0].
• Add clearActiveProviderProfile(): records the sentinel as the active id, clears the managed provider env from the current session (so the switch is immediate, no restart), and removes the startup profile mirror file.
• Surface a Use Anthropic (built-in) choice in the /provider menu, shown whenever the current provider is not Anthropic. Selecting it switches mid-session and emits a provider-switch system reminder.

Saved profiles are preserved for re-selection — this only changes which one is active.

Test plan

• bun test src/utils/providerProfiles.test.ts — added coverage: sentinel resolves to undefined while saved profiles persist; managed env cleared from session.
• bun test src/commands/provider/provider.test.tsx
• bun run build
• node dist/cli.mjs --version (smoke)

[0xghost42]

Heads up on the red smoke-and-tests: the 4 failures (getAttributionTexts coAuthoredBy, the two auto-compact cooldown tests, and breaker metadata tracking callback publishes a fresh object) are pre-existing on main, not from this change.

I checked out a clean upstream/main and ran the full suite — same 3 cooldown/breaker failures reproduce there with zero diff applied (the attribution one is order-dependent and only trips in the full CI run). On this branch:

• bun test src/utils/providerProfiles.test.ts → 76 pass (3 new)
• bun test src/commands/provider/provider.test.tsx → 25 pass
• the four flaky files pass in isolation
• bun run build + node dist/cli.mjs --version green

The diff only touches provider profile state (providerProfiles.ts, provider.tsx) — nothing in the attribution/auto-compact/breaker paths. Happy to rebase once main is green if that helps the check go through.