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.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/contracts/src/ws-frames.ts

@@ -124,6 +124,12 @@ export const MessageCompleteFrame = z.object({
   // status:'complete' transition) is dropped.
   model: z.string().nullable().optional(),
   metadata: OpaqueObject.nullable().optional(),
+  // F1 (D-8): the terminal status of the assistant message. Absent on the native
+  // BooChat path (reducer defaults to 'complete'); the BooCoder dispatcher stamps
+  // it 'cancelled' on a user Stop / stall and 'failed' on a thrown error so the
+  // web reducer can render a muted "Stopped" / failed state without a new frame
+  // type. Optional → fail-closed publishFrame must keep, not strip, it.
+  status: z.enum(['complete', 'cancelled', 'failed']).optional(),
 });
 
 export const UsageFrame = z.object({