fix(subagents): keep a settled run findable by the id it was given

[brynne8]

What breaks today

A parent dispatches a background Agent run, the child finishes, and the parent's next get_subagent_result({ sessionId }) — for the very id the Agent call just returned — answers "Subagent not found". The parent has no way to check that claim, so it reasonably concludes the child's session was deleted, and either re-does the work or reports a loss that never happened.

Why it happens

Three lookups in get() all come up empty at once, right after a run settles:

1. Settling deletes the run from the live registry (getSubagentRuns().delete(...) at the six settle points).
2. The child's AgentSessionWrapper is released once the child goes idle, so there is no live wrapper to read from.
3. dependencies.resolveSessionPath(sessionId) can still miss a session that settled moments ago.

With all three gone, nothing answered for an id this process had handed out itself.

The fix

• A settled run moves to getSettledSubagentRuns() — a bounded map (64 entries, insertion order) — instead of vanishing.
• get() consults it only after both the live wrapper and the session-file read have failed.

What the fallback deliberately cannot do:

• Shadow a real file read. A fresh read of the session file always wins; the record only fills the hole the lookup leaves.
• Invent an unknown id. An id that was never run is still reported as unknown (get returns null).
• Serve a superseded result. resume re-registers the run live, and a re-settled resume overwrites the record.

Test evidence

The new test is behavioural: it resumes/settles a run against the real controller, then takes away both the wrapper (getSession returns nothing, resolveSessionPath returns null) and asserts get() still returns the completed result with the resumed run's text; it also asserts an id nothing ever ran is still null. No source-text assertions.

• Fails against current main (test kept, fix stashed):
  • bun test lib/subagent-runtime.test.mjs → 14 pass / 1 fail (AssertionError: actual: undefined, expected: "completed")
  • node --experimental-strip-types --test lib/subagent-runtime.test.mjs → same single failure
• Passes with the fix, both runners: 15 pass / 0 fail
• Full suite npm test: 2627 tests, 2626 pass, 0 fail, 1 skipped
• bun x tsc --noEmit → exit 0; bun run lint (eslint) → exit 0