feat: post-review backlog hardening (cancel/parser/stall/history/9502)

Five independent items from the post-review backlog. F1: Stop on an external
agent task now aborts the running child via a per-task AbortController registry
reachable from the cancel route, and finalizes the assistant message as
cancelled (fixing two latent bugs — catch blocks left the message streaming,
and warm success-paths wrote complete on an aborted turn); warm pools/worktrees
are preserved and the native path is unchanged. F2/F3: prune the tool-call
parser to its two load-bearing exports (unexport eight zero-caller symbols, add
a gate test for the <invoke>-as-text fallback) and route placeholder-rejection
logging through pino. F6: a 90s per-chunk stall-timeout wraps native inference's
fullStream via AbortSignal.any so a hung stream finalizes the message instead of
hanging — no retry (a pure classifyStreamError helper is added). F7: a read-only
view_session_history MCP tool (newest-N, chronological). F9: retire the unused
apps/coder/web :9502 fallback SPA, keeping every API/WS/health/MCP route.

apps/web/src/api/types.ts

@@ -442,6 +442,11 @@ export type WsFrame =
       // cap-hit sentinels (and any future stamped-on-complete metadata) flow
       // to the client without a refetch.
       metadata?: MessageMetadata | null;
+      // F1 (D-8): terminal status of the assistant message. Absent on the normal
+      // path (reducer defaults to 'complete'); the BooCoder dispatcher stamps it
+      // 'cancelled' on a user Stop / stall and 'failed' on a thrown error so the
+      // reducer renders a muted "Stopped" / failed state — no new frame type.
+      status?: 'complete' | 'cancelled' | 'failed';
     }
   // v1.12.2: live throughput frame, published mid-stream every ~500ms with
   // the latest token + ctx counts so ChatThroughput can render tok/s and