feat(kimi-code): add experimental OS keychain credential storage

[7Sageer]

Related Issue

N/A — no tracking issue; the motivation is described below.

Problem

OAuth credentials (managed provider logins and MCP server grants) are stored as plaintext JSON files under ~/.kimi-code/credentials. File permissions (0600) are their only protection — anything running as the user can read long-lived refresh tokens, and secrets like these belong in the OS credential vault.

What changed

Adds experimental OS keychain credential storage, opt-in via KIMI_CODE_EXPERIMENTAL_KEYRING=1 (with KIMI_DISABLE_KEYRING=1 as a kill switch):

• packages/oauth stays pure TypeScript and gains a KeyringTokenStorage that stores each token in the OS keychain (macOS Keychain, Windows Credential Manager, Linux Secret Service) using the exact same wire payload as the file store. Backend selection is gated by the opt-in flag, the kill switch, backend registration, and a runtime capability probe; any keyring call that throws degrades the backend stickily to the file store, so a locked or disappearing keychain never breaks login state.
• Plaintext tokens written by older builds migrate into the keychain on first read via lock-free compare-and-delete, and reads reconcile against the file store so a backend flip-flop can never shadow a newer token or resurrect a revoked one. Keychain services are namespaced per credentials directory, keeping the same profile isolation as the file backend.
• apps/kimi-code loads the @napi-rs/keyring native binding at bootstrap and registers it as the backend, wired into the SEA native-asset pipeline and bundle checks; a load failure only warns and keeps the file store.
• packages/agent-core-v2 gives the MCP OAuth grant store the same keychain-backed treatment, selected when a backend is registered and the gate opts in.
• Docs: env-vars and data-locations pages (en + zh).

The opt-in gate is temporary; flipping the default once the rollout is proven is a one-line follow-up.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue (none exists — the problem is described above).
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

🦋 Changeset detected

Latest commit: c566aa9

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Minor

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[pkg-pr-new]

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npx https://pkg.pr.new/@moonshot-ai/kimi-code@c566aa9

commit: c566aa9

[github-actions]

❌ Nix build failed

Hash mismatch in pnpmDeps:

	Hash
specified	sha256-NDcCQ5vxsGaSdJ3U0bvq2RkXKwrYTI7/8zZn/x1fvJ8=
got	sha256-YYfWyLv+CkUuPp05iUIJ+zOAU8TSQHvJFnvxyj9HeMI=

Please update flake.nix with the got hash.