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>

apps/coder/src/services/cancel-registry.ts

@@ -0,0 +1,50 @@
+/**
+ * F1 — per-task abort registry. A Stop on an external-agent task must reach the
+ * in-flight run and abort its child / prompt. Each external run-function registers
+ * its per-turn AbortController here keyed by task id; the cancel route calls
+ * `cancel(taskId)` to fire it; the run-function's `.finally` deletes the entry.
+ *
+ * Idempotent by construction:
+ *   - `cancel()` on an already-aborted controller no-ops (AbortController.abort()
+ *     is idempotent) → a rapid double-Stop is safe.
+ *   - `cancel()` on an unknown / already-finished task returns false → a
+ *     cancel-after-natural-exit (entry already deleted) and a Stop on a native
+ *     boocode task (never registered) are both safe no-ops.
+ *
+ * Pure (no DB / child / IO) so the abort wiring + idempotency contract is
+ * unit-testable in isolation — mirrors the turn-guard / lifecycle-decisions
+ * pure-helper precedent.
+ */
+export interface CancelRegistry {
+  /** Create + store an AbortController for this task, returning it for the run. */
+  register(taskId: string): AbortController;
+  /** Abort the task's in-flight run. Returns false when no controller is registered. */
+  cancel(taskId: string): boolean;
+  /** Drop the task's entry (called from the run's `.finally`). No-op if absent. */
+  delete(taskId: string): void;
+  /** Whether a controller is currently registered for this task. */
+  has(taskId: string): boolean;
+}
+
+export function createCancelRegistry(): CancelRegistry {
+  const controllers = new Map<string, AbortController>();
+  return {
+    register(taskId) {
+      const ac = new AbortController();
+      controllers.set(taskId, ac);
+      return ac;
+    },
+    cancel(taskId) {
+      const ac = controllers.get(taskId);
+      if (!ac) return false;
+      ac.abort();
+      return true;
+    },
+    delete(taskId) {
+      controllers.delete(taskId);
+    },
+    has(taskId) {
+      return controllers.has(taskId);
+    },
+  };
+}