refactor(meta): deduplicate extension internals, pin failure paths with tests

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What

A behavior-preserving cleanup of extensions/meta.ts (789 → 761 lines), preceded by a new test file that pins the paths being restructured.

Commit 1: test(meta). Adds tests/meta-failures.test.ts (49 tests). It covers behavior the existing suites didn't: slow_down back-off, denied/expired/unknown device-login errors, default interval and expiry, request shapes and progress messages, exact mint-failure messages, catalog refresh fallbacks (offline, no key, HTTP error, empty catalog, abort, failed persistence), the bundled model table, and MODEL_API_KEY/META_API_KEY mirroring. All of these pass against the unmodified extension.

Commit 2: refactor(meta).

• Five near-identical bundled-model literals → one museFallbackModel() factory plus shared cost constants.
• The ": ${detail}" suffix was copy-pasted in five places → detailedError().
• loginMeta (80 lines) split into startDeviceAuthorization and pollForIdentityToken.
• The "cached or fallback" tail repeated four times in refreshMetaModels → cachedOrFallbackModels(); credentialApiKey and fetchMetaCatalog extracted.
• Thinking-level maps built in a loop; shared Muse compat and default limits; toProviderModels gates the one fallback it needs instead of the whole list per catalog entry.
• responseBody is generic and reuses asRecord (duplicate removed); env mirroring extracted to mirrorApiKeyEnv().
• AGENTS.md: one line noting the new test file.

Why

meta.ts had a lot of copy-paste (model literals, error formatting, fallback handling) that made each change touch many places. The existing tests didn't cover the failure paths being restructured, so the safety net came first.

Reviewer notes

• Verification: bun run typecheck is clean. bun test gives 86 pass, 3 skip, 0 fail. The skips are the optional live probes. I ran with live credentials blanked and HOME redirected, because the live probes make real billable requests when a credential resolves.
• Mutation check: I injected 12 deliberate bugs into the refactored code. Nine were caught by the tests; the other three are equivalent mutations (e.g. swapping which env key wins in ??=, which can't change behavior).
• Removed export: metaFallbackCost is gone. Its only caller was the media module dropped in v0.6.0 and nothing here references it, but it was technically a public export.
• Still one file: I did not split meta.ts into modules. tests/package.test.ts and the branch contract in AGENTS.md require extensions/ to contain only meta.ts.
• Contract names preserved: gateMuseMaxEffort(), applyMetaResponsesCacheHints(), MUSE_USER_AGENT_MODEL_IDS, and the Pi 0.83/0.84 refresh-context handling.
• No CHANGELOG entry, since there is no user-visible change; that's left to the release commit.

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