`parent_entry_id` + `message_type` on the SQLite message table

[Hmbown]

Problem. Sessions are linear. No fork, no try-this-instead, no compare-two-replies, no place for an extension to persist state without polluting the LLM context.

Shape.

• Add columns to the message table in crates/state/src/lib.rs:
  • parent_entry_id TEXT (null on first message = legacy linear)
  • current_leaf_id TEXT pointer on the thread row
  • message_type TEXT with values user, assistant, tool_result, custom, custom_message
• Add StateStore::fork_at_message(message_id) → returns new leaf id.
• DDL migration; idempotent.

Acceptance.

• Migration runs cleanly on existing sessions.
• Unit test forks a 5-message session and proves both leaves are queryable independently.
• Existing reads return all legacy messages unchanged (parent_entry_id null behaves as the linear default).

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Existing issues this resolves on PR merge

Partially addresses:

• Partially addresses Feature Request: Improve Fork UX #576

Related:

• Related: Sub-agent runtime sub-issue: checkpoint and continue child work across turns #2029

When opening a PR for this issue, the PR body must include the Closes #N lines for every issue above that this PR fully resolves, so they auto-close on merge.

Slate item 9/18. Tracking doc: .private/issue-slate.md (maintainer-only).