feat(coding-agent/session): typed CompactionCancelledError sentinel and CompactionOutcome

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

packages/coding-agent/src/session/compaction/errors.ts

@@ -0,0 +1,31 @@
+/**
+ * Compaction error types.
+ *
+ * `CompactionCancelledError` is the canonical signal raised when a compaction
+ * is explicitly aborted — operator Esc, extension hook returning `cancel`,
+ * programmatic `session.abortCompaction()` call, or any other deliberate
+ * abort source. Downstream callers (e.g. `executeCompaction`) discriminate
+ * cancellation from other failures via `instanceof CompactionCancelledError`
+ * rather than introspecting error messages or `name` fields — the typed
+ * sentinel makes classification source-agnostic and refactor-stable.
+ */
+
+export class CompactionCancelledError extends Error {
+	readonly name = "CompactionCancelledError" as const;
+
+	constructor(message = "Compaction cancelled") {
+		super(message);
+	}
+}
+
+/**
+ * Outcome of a compaction attempt, surfaced by `CommandController.executeCompaction`
+ * so callers (e.g. the plan-mode approval flow) can distinguish a deliberate abort
+ * from an unrelated failure.
+ *
+ *   "ok"        — compaction completed; transcript was summarized.
+ *   "cancelled" — `CompactionCancelledError` was raised. Operator Esc, extension
+ *                 hook, programmatic abort — all source-agnostic.
+ *   "failed"    — any other rejection from `session.compact()`.
+ */
+export type CompactionOutcome = "ok" | "cancelled" | "failed";

packages/coding-agent/src/session/compaction/index.ts

@@ -4,4 +4,5 @@
 
 export * from "./branch-summarization";
 export * from "./compaction";
+export * from "./errors";
 export * from "./utils";