feat(common): add rollback_migrations helper to agentd_common::storage

[geoffjay]

Adds rollback_migrations<M>(db_path, steps) to agentd_common::storage, following the same pattern as apply_migrations and migration_status. steps=None rolls back all applied migrations; steps=Some(n) rolls back the n most recent.

Closes #1123

[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(common): add rollback_migrations helper to agentd_common::storage

LGTM — cannot self-approve, posting as comment.

Stack note

This branch (issue-1123) is the base of the entire core migration stack. The chain above it is:

• #1131 (issue-1124): expose helpers from agentd_core::lib
• #1132 (issue-1125): clap CLI with migrate subcommand
• #1133 (issue-1127): integration tests for the CLI
• #1134 (issue-1126): xtask registration

This PR must merge first; everything else in the stack depends on rollback_migrations being available in agentd_common::storage.

Base branch observation (non-blocking): This PR targets feature/core which itself targets main. CLAUDE.md specifies all feature work should PR into feature/autonomous-pipeline, not main. feature/core (#1135) appears to be a legitimate collecting branch for core service work, so this is likely intentional — but worth confirming the feature/core → main targeting is deliberate before the final merge.

What was verified

• Pattern match: structure is identical to apply_migrations — create_connection then the SeaORM migrator call then Ok(()). ✓
• SeaORM API: MigratorTrait::down(&db, steps) is the correct call; None = roll back all, Some(n) = roll back n most recent. Matches the doc comment exactly. ✓
• Error propagation: both create_connection and M::down use ? — no unwrap anywhere. ✓
• No connection leak: db is local and dropped at end of function. ✓
• Placement: inserted between apply_migrations and migration_status — logical ordering of the three migration operations. ✓
• Docs: steps semantics documented inline with the bullet-point convention used elsewhere. ✓
• Scope: one function, 13 lines, no other changes. ✓

Non-blocking suggestion

A unit test for rollback_migrations would complete the test coverage alongside the existing test_create_connection / test_create_connection_creates_file tests. The function has no internal logic to test beyond the delegation, so the integration tests in #1133 provide adequate coverage — but a direct test like:

#[tokio::test]
async fn test_rollback_migrations_is_callable() {
    // smoke-test: apply then roll back using a concrete Migrator
}

would guard against future signature drift. Non-blocking; can be a follow-up.