fix(summarizer): keep Codex summaries on the Codex route

[HengYangDS]

Problem

When Codex app-server summarization fails, summarizeConversation currently falls through to the Claude Agent SDK's transcript-text path. A Codex exchange without a forkable session ID also takes that Claude path. This silently changes provider, authentication and potential cost instead of reporting a Codex summary failure.

Change

• Route every Codex exchange through the Codex branch only. Require a session ID for the existing ephemeral, read-only thread/fork mechanism.
• Propagate a Codex app-server error, or a missing session ID, rather than invoking Claude. Existing callers can record a retryable summary failure while indexing exchanges independently.
• Update current user guidance; the Claude fallback model remains relevant only for Claude summaries. This PR does not select or rewrite any Codex conversation model.

Verification

• Two regression tests failed on upstream main: an unavailable Codex binary and a Codex transcript without a session ID both incorrectly returned a Claude summary. Both pass after the change and assert the Claude SDK is never called.
• Full local suite: 352 tests passed across 64 files. npm run build, bundle syntax, and a redacted commit-range secret scan passed.
• Local tests reused pre-existing development dependencies and a local model cache; a fresh local npm install did not complete inside a 300-second bound. This is not a cold-install claim. The fork PR's Node 22/24 CI is the cold-install gate.

Scope and risk

No cross-provider fallback is attempted for Codex. A Codex transcript lacking a session ID remains indexable but has no generated summary until a safe Codex-native transcript path exists. Claude-session summarization behavior is unchanged. The large generated bundle diff reflects upstream's unpinned transitive dependency layout, not host-specific code or personal policy.

Additional local Node matrix (2026-09-26)

On macOS arm64, this current PR head passed npm run build, node --check dist/mcp-server.js, and the complete test suite on Node 22.23.3 and Node 24.21.0 (352/352 tests on each). Each run used a fresh source snapshot and the corresponding per-Node native dependency tree, with a pre-seeded local embedding-model cache and an isolated OMP_HOME. The dependency trees were installed from PR #179 (all five PRs have the same package.json); this is not a separate empty-cache install for each PR, an Ubuntu result, or an upstream CI pass.