Pre-manual e2e hardening

[rogerchappel]

Summary

• isolate Playwright e2e runs on port 3100 with a clean PGlite data dir
• make PGlite bootstrap split multi-statement migrations and repair newer columns on startup
• return stable API conflict responses for duplicate agents and task error hashes
• allow task PATCH assignment validation to resolve callsign-style agent refs

Verification

• pnpm typecheck
• pnpm lint:check (warnings only, existing)
• pnpm test
• pnpm build
• pnpm release:check
• git diff --check
• pnpm test:e2e (43 passed, 1 skipped, 7 failed)
• targeted e2e: duplicate callsign, task errorHash dedupe, task lifecycle, task assignment

Known remaining e2e gap

The remaining 7 e2e failures are all caused by seeded demo agents/projects/tasks not being readable from the first signed-up user's personal workspace. Fixing that requires an explicit auth/workspace access decision: either grant the first user ownership/membership of seeded demo company data, or change the demo seed/onboarding flow.