Introduce `CompactionCancelledError` and `CompactionOutcome` ("ok" |
"cancelled" | "failed") so callers can discriminate user-driven aborts
from generic failures via `instanceof`, instead of inspecting error
messages or `AbortError`-name strings.
`AgentSession.compact()`'s two abort-rejection sites now throw the
typed sentinel; the model-call wrapper normalizes AbortError-shaped
rejections to the sentinel only when the compaction's abort signal
is actually set, preserving every other exception unchanged so real
compaction bugs are not silently relabeled as cancellations.
`CommandController.executeCompaction` and `handleCompactCommand`
return `Promise<CompactionOutcome>`; the catch classifies via
`instanceof CompactionCancelledError`. Existing callers (`/compact`,
loop runner, auto-compact) ignore the return value — non-breaking.
Op: extend
cognitive
bad4c133e7