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/Dockerfile

@@ -7,7 +7,6 @@ WORKDIR /build
 COPY package.json pnpm-workspace.yaml pnpm-lock.yaml tsconfig.base.json ./
 COPY apps/server/package.json ./apps/server/
 COPY apps/coder/package.json ./apps/coder/
-COPY apps/coder/web/package.json ./apps/coder/web/
 
 RUN pnpm install --frozen-lockfile
 
@@ -16,7 +15,6 @@ COPY apps/server ./apps/server
 RUN pnpm -C apps/server build
 
 COPY apps/coder ./apps/coder
-RUN pnpm -C apps/coder/web build
 RUN pnpm -C apps/coder build
 
 RUN pnpm deploy --filter=@boocode/coder --prod --legacy /out/coder
@@ -27,7 +25,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends ripgrep git ope
 WORKDIR /app
 
 COPY --from=builder /out/coder ./
-COPY --from=builder /build/apps/coder/web/dist ./web
 
 ENV NODE_ENV=production
 EXPOSE 3000