[#17e] `jcode session migrate` subcommand: idempotent v1 → tree-aware schema upgrade

[quangdang46]

[#17e] jcode session migrate subcommand: idempotent v1 → tree-aware schema upgrade

Sub-issue of #17. The user-facing command that upgrades existing JSONL session files to the new tree-aware / v2-sidecar-compatible schema.

Hard dependencies

• Session tree branching: /tree, /fork, /clone in-place navigation #2 (tree branching defines the target schema).
• #17a (sidecar must exist so migrated sessions can be indexed atomically).

Subcommand surface

• jcode session migrate (default: all sessions in ~/.jcode/agent/sessions/).
• jcode session migrate --session <id-or-path> — single file.
• jcode session migrate --dry-run — report what would change without writing.
• jcode session migrate --all — same as default; explicit form.

Behavior

• For each session file:
  1. Detect schema version (top-line metadata or absent → infer v1).
  2. If already current → skip with up-to-date.
  3. Else rewrite using the temp-then-rename helper from #17b.
  4. Re-index in the sidecar DB from #17a.
• Idempotent: running twice produces the same result.
• Atomic per file: a migration failure on one file does not touch the rest; previous version is preserved as <file>.v1.bak.

Output

• Concise summary: Migrated 13 sessions, 0 skipped, 1 failed (see ~/.jcode/logs/migrate.log).

Acceptance criteria

• migrate --dry-run writes nothing.
• migrate --all on a fresh dir does nothing the second time.
• A linear v1 session round-trips to the new schema with identical visible content.

Cross-references

• Parent: jcode session migrate: session storage migration & v2 sidecar fast-path for large sessions #17
• Siblings: #17a, #17b, #17c, #17d
• Related: Session tree branching: /tree, /fork, /clone in-place navigation #2 (schema), Add general-purpose jcode doctor diagnostic command #8 (doctor --only sessions should suggest running this when stale files are detected).

Reference

• pi_agent_rust → pi migrate subcommand.