Issue for this PR

No linked issue.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds timeline-backed session history. Forks reference a frozen prefix, committed undo selects a new timeline, and reads use indexed message ranges. Shared history survives deletion of its originating session. Active forks omit unfinished messages and materialize only the necessary settled suffix.

The transactional migration gives every existing session a root timeline and rebuilds the session/message tables while preserving their data and dependent rows. V1 imports create timelines directly; the temporary opencode-next.db importer is removed.

Clients keep flat session message lists. Undo handling uses message order rather than lexical IDs, and snapshot imports handle shared-ID collisions. Public API/event schemas and event versions are unchanged; settled-message selection happens inside the fork projection transaction.

How did you verify your code works?

• Workspace pre-push typecheck: 33 tasks passed.
• Core session/instruction/migration suites: 650 tests passed.
• TUI: 80 tests; client: 24; server: 5; protocol: 8; schema event manifest: 7—all passed.
• Temporary checks exercised populated migration with FK enforcement, nested and active forks, pagination, independent undo, source deletion, and overlapping snapshot imports.
• Client generation and migration-schema consistency checks passed.
• One unrelated schema hygiene test fails on both this branch and v2 because of existing Schema.Any usage.

Screenshots / recordings

Not applicable.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR