feat(store): support invariant-safe session forks

[ZhiXiao-Lin]

Summary

• add SessionSnapshotV1::fork_for_session for rebinding a complete persisted generation to a new session id and workspace
• preserve artifacts, traces, run ids, verification reports, and child-session ids
• rebind top-level run ownership and non-empty subagent parent ownership so the fork remains loadable under existing invariants
• validate both the source and resulting snapshot, and reject empty destination session ids

Why

The CLI recovery work for /fork, isolated worktree forks, and conflict-safe /rewind needs one canonical Core operation. Copying only SessionData would silently discard or mis-own the rest of the atomic session snapshot.

Validation

• cargo fmt --all -- --check
• cargo test -p a3s-code-core snapshot_fork_rebinds_every_top_level_session_owner
• git diff --check

A focused dependent CLI PR will follow separately after this Core seam is available.