fix: mark informational chunks with metadata

[kocaemre]

Summary

• tag SDK informational notices with _meta.claudeCode.kind = "informational"
• include the notice level so clients can separate hook/status notices from model text
• keep the existing rendered text unchanged for clients that ignore _meta

Fixes #1042

Test plan

• npm test -- --run src/tests/acp-agent.test.ts -t informational
• npm run build
• npm run check
• IS_SANDBOX=1 npm run test:run
• git diff --check

[kocaemre]

Rebased this branch onto current main (7c66108, zod import fix) and re-verified the rebased head 6dd450e.

Verification:

• npm test -- --run src/tests/acp-agent.test.ts -t informational — 1 passed, 461 skipped
• npm run build — passed
• npm run check — ESLint passed, Prettier check passed
• IS_SANDBOX=1 npm run test:run — 1045 passed, 27 skipped
• git diff --check upstream/main..HEAD — clean

gh pr checks still reports no checks on main, so there is no CI failure to act on after the rebase.