feat: add durable agent session archives

[roackb2]

Summary

• add service-only archive content and manifest-head tables scoped to an existing user session
• add an atomic append RPC that serializes concurrent compaction writes and rejects partial or duplicate state
• document the server-only storage boundary and add migration contract coverage

Verification

• focused migration contract tests pass
• full migration chain applied successfully in an isolated local Supabase stack
• live PostgreSQL checks passed for append, duplicate rejection, malformed-record rollback, cross-user rejection, direct-write denial, and parent-delete cascade

The GitHub lifecycle reached browser E2E before the repository's 15-minute workflow timeout cancelled the job. All earlier lifecycle steps passed; the database acceptance above was also run directly against the full migration chain.

Integration order

This schema is the prerequisite for the paired agent-server adapter in roackb2/slidex-agent-server#24.

1. Merge and deploy this migration to the target Supabase project.
2. Confirm the archive tables and append_agent_session_archive RPC exist.
3. Deploy the paired server change.
4. Enable HEDDLE_SESSION_STORAGE=supabase only after the schema is live.

The migration is the authoritative schema; no manual table design or hand-created database objects are required.