test: add agent lifecycle browser regression

[roackb2]

What changed

• add a deterministic Playwright regression for the SlideX agent lifecycle
• verify two turns reuse the same product session and the MotionDoc produced by the previous turn
• verify refresh restores visible conversation history
• verify New conversation clears chat state while retaining the current deck for the next fresh session
• pin Playwright 1.61.1 and add a focused GitHub Actions job with failure artifacts
• document the boundary between the editor protocol fixture and real server lifecycle tests

Why

The lifecycle behavior in #6 was manually browser-verified, but the MVP needs a committed regression that can catch storage, hydration, streaming, and reset regressions across a real browser reload. Component-only tests would not exercise those boundaries.

The route fixture intentionally owns deterministic HTTP/SSE responses only. It does not reproduce Heddle run coordination or server session policy; those remain covered in the companion server repository.

Dependency and rollout

• stacked on and intended to merge after #6
• the production agent flag remains default-off; Playwright enables it only in its test web server
• active-run nonzero cursor restoration remains a follow-up after Heddle #238 is consumable

Validation

• clean npm ci
• npm run test:agent (7 passed)
• npx tsc --noEmit
• npm run lint (0 errors; 11 existing warnings)
• npm run test:agent:e2e from a cold start (1 passed)
• feature-enabled npm run build
• independent dev-server browser check: meaningful page, expected controls, no error overlay, no captured console errors

npm audit --omit=dev continues to report the existing Next/Geist PostCSS advisory (3 moderate); this PR adds only a pinned dev dependency and does not change the production dependency finding.