feat(core): expose apply/status/rollback migration helpers from lib.rs

[geoffjay]

Adds apply_migrations_for_path, migration_status_for_path, and rollback_migrations_for_path to agentd_core lib.rs, following the pattern established by agentd-orchestrator. Required by the migrate CLI subcommand.

Closes #1124

[geoffjay]

This change is part of the following stack:

• feature/core #1135
  • feat(common): add rollback_migrations helper to agentd_common::storage #1130
    • feat(core): expose apply/status/rollback migration helpers from lib.rs #1131 ◀
      • feat(core): add clap CLI with migrate subcommand to agentd-core binary #1132
        • test(core): add integration tests for agentd-core migrate subcommand #1133
      • feat(xtask): register agentd-core in DB_SERVICES for migration management #1134

_{Change managed by git-spice.}

[geoffjay]

Review: feat(core): expose apply/status/rollback migration helpers from lib.rs

LGTM — cannot self-approve, posting as comment.

Stack note

This branch (issue-1124) is stacked on PR #1130 (issue-1123), which adds rollback_migrations to agentd_common::storage. That parent must merge before this one. This PR is itself the parent of #1132 (migrate CLI) and #1134 (xtask registration) — both of which depend on these three function signatures being exactly as defined here.

What was verified

• Pattern match: all three helpers follow the identical shape used by agentd-orchestrator, agentd-memory, and agentd-communicate (confirmed in earlier stack reviews). ✓
• Signatures:
  • apply_migrations_for_path(&Path) -> anyhow::Result<()> ✓
  • migration_status_for_path(&Path) -> anyhow::Result<Vec<(String, bool)>> ✓
  • rollback_migrations_for_path(&Path, Option<u32>) -> anyhow::Result<()> — None = all, Some(n) = latest n ✓
• Delegation: each function delegates to the corresponding agentd_common::storage generic parameterised on migration::Migrator. No logic, no state — pure pass-through. ✓
• Docs: each function has a doc comment; rollback_migrations_for_path documents the steps semantics explicitly. ✓
• Scope: single file, 27 lines added, no production logic changed. ✓