fix(prompt-hook): skip Claude Code task notifications (#1832)

[danusha2345]

Symptom

Claude Code's UserPromptSubmit hook also fires for the <task-notification>…</task-notification> blocks the host injects as user messages when a background Agent finishes. Those blocks are long machine text (task IDs, toolu_* IDs, scratchpad paths, an agent's prose summary about code), so they pass the structural gate on the merits and the whole blob is sent to codegraph_explore as the query. Reported cost: 3.5 s median / 7.4 s max on the blocking path, on text the user never typed (36% of the hook's firings in the reporter's history).

Fix

• isTaskNotification(prompt) in src/directory.ts: true only when the entire prompt (modulo leading/trailing whitespace) is one <task-notification>…</task-notification> envelope.
• codegraph prompt-hook returns immediately after parsing stdin when the predicate matches — before telemetry, the gate, project lookup or any explore work. No output is written.
• A prompt that merely mentions the marker, or has any text around the envelope, is still treated as a user prompt (no arbitrary XML-looking prompts are skipped).
• CHANGELOG entry under [Unreleased] → ### Fixes.

Verification

• npx tsc --noEmit -p tsconfig.json → exit 0.
• npx vitest run __tests__/frontload-hook.test.ts → 42 passed (new describe('system task notifications (#1832)') covers the envelope with/without surrounding whitespace and multi-line body, and the negative cases: marker inside a question, trailing text, other tag, plain prompt).
• Smoke test of the built dist/bin/codegraph.js prompt-hook from an indexed repo: a <task-notification> payload (Claude Code's {"prompt","cwd"} JSON on stdin) prints 0 bytes and exits 0 in ~0.26 s; a normal structural prompt still prints the <codegraph_context> block.

Fixes #1832

🤖 Generated with Claude Code