flaky: TestSubprocessClaudeBackgroundStatesAndCleanup/timeout races the record write

[rianjs]

TestSubprocessClaudeBackgroundStatesAndCleanup/timeout in internal/llm (now internal/llmadapters after #499) is flaky. Two independent failures on 2026-07-11, both on code that did not touch the LLM subprocess path:

1. CI static-smoke job on PR test(configcmd): byte-exact config show --json goldens (477.1) #495 (test-only config goldens): the subtest failed; the full test job passed on the same commit; a rerun passed.
2. Local full-suite run during refactor(runlifecycle): extract pure run-lifecycle helpers (475.3) #498 verification:

--- FAIL: TestSubprocessClaudeBackgroundStatesAndCleanup (0.34s)
    --- FAIL: TestSubprocessClaudeBackgroundStatesAndCleanup/timeout (0.05s)
        subprocess_test.go:388: ReadFile(record): open .../TestSubprocessClaudeBackgroundStatesAndCleanuptimeout.../001/records.jsonl: no such file or directory

Immediately re-running the test (-count=2) passed.

The shape of the failure — the timeout subtest killing the fake subprocess before it has flushed records.jsonl — suggests a race between the timeout-triggered cleanup and the helper process's record write, i.e. the test asserts on a file the child may not have created yet when the timeout fires quickly. Worth either synchronizing the fake's record write with the parent's expectation or tolerating a missing record file in the timeout case, whichever matches the intent of the assertion.