feat(lineage): write fork lineage back into Codex rollouts

[malihashar]

Closes #82.

Problem

Meta::lineage is read-only today — forked_from_id is read from Codex rollouts, but nothing ever writes it back. A session copied via continue <id>#range or crop loses its parent link entirely once written back out.

Fix

Codex's from_common now writes Fork lineage as session_meta.forked_from_id for same-harness copies. Scope is intentionally narrow:

• Written: same-harness Codex copies (Fork, UUID parent).
• Not written: Spawn/Continue relations (no matching native field without changing the session's meaning), non-UUID parents, and cross-harness copies (Lineage::parent only makes sense within one harness).
• Other harnesses (pi/Campfire, OpenCode, Claude Code) still don't write lineage — their native fields mean something different or aren't reachable from from_common. Covered by a byte-for-byte cross-harness test.

Open questions (flagged in #82)

• Should txcript's own copies be tagged Fork or Continue?
• Is Fork accurate for an edited crop copy, not just a plain continue?
• Cross-harness provenance isn't representable under the current model and would need a separate design decision.