Log the start of model reasoning in background job logs

[cubicj]

Summary

Background job logs (state/<slug>-<hash>/jobs/<id>.log) go silent during reasoning stretches. Every other item type already logs a start line via describeStartedItem in plugins/codex/scripts/lib/codex.mjs: commandExecution ("Running command: ..."), fileChange ("Applying N file change(s)."), mcpToolCall, dynamicToolCall, collabAgentToolCall, and webSearch. reasoning is the only started item type that falls through to default: return null, so nothing is written and anyone tailing a job log cannot tell whether the model is thinking or the job is stuck.

Change

Add a reasoning case to describeStartedItem that emits Thinking. with phase: null. The message follows the existing period-terminated declarative style ("Turn completed.", "File changes completed."). phase stays null deliberately: none of the existing phase values (starting/running/editing/investigating/verifying/reviewing/finalizing) describes reasoning, and inventing a new phase would widen the change beyond logging.

Verification

• Live-verified against plugin v1.0.6 (WSL2, Codex CLI 0.145.0-alpha.2): with this case patched into the installed plugin, job logs gain a real-time [<iso-ts>] Thinking. line at the start of each reasoning block. This confirms the app-server delivers item/started for reasoning items; the stock plugin currently drops them.
• npm test: 91/91 passing. The fake-codex fixture only emits completed reasoning items, so no existing test output changes.
• npm run build: clean.

Noise consideration

Reasoning blocks complete every few seconds during long thinking stretches, so this adds one short line per block rather than one line per turn. Consumers that want a quieter view can collapse consecutive markers; the downstream log watcher @cubicj/codex-watch 0.3.0 already renders them as a single collapsed "Thinking..." indicator.

Related

• Add observer and worktree features with multi-root support #359 adds a structured .events.jsonl stream via /codex:observe; this change instead improves the existing .log files that current tooling tails, and the two are complementary.