test(core): add integration tests for agentd-core migrate subcommand

[geoffjay]

Adds crates/core/tests/migrate_cli.rs with 4 scenarios exercised via std::process::Command against isolated temp databases: up+status round-trip, status on empty DB, down+status rollback, and non-TTY down without --yes exits non-zero.

Closes #1127

[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 ◀

_{Change managed by git-spice.}

[geoffjay]

Review: test(core): add integration tests for agentd-core migrate subcommand

LGTM — cannot self-approve, posting as comment.

Stack note

This branch (issue-1127) is stacked on:

• #1132 (issue-1125): feat(core): add clap CLI with migrate subcommand to agentd-core binary — still open
• #1131 (issue-1124): feat(core): expose apply/status/rollback migration helpers from lib.rs — still open
• #1130, #1129 below that

The entire upstack must merge before this lands. The tests depend on the migrate subcommand and --db-path flag introduced in #1132.

What was verified

• Binary target: [[bin]] name = "agentd-core" is declared in Cargo.toml → env!("CARGO_BIN_EXE_agentd-core") resolves correctly at test time. ✓
• tempfile dev-dep: present at tempfile = "3.14" in [dev-dependencies]. ✓
• --db-path flag: all three MigrateAction variants (Status, Up, Down) expose #[arg(long)] db_path: Option<PathBuf>. ✓
• Output string assertions cross-checked against main.rs:
  • "up to date" / "applied" — emitted by run_migrate_up ✓
  • "0 pending" — appears in the summary line "N applied, 0 pending" ✓
  • "0 applied" (empty DB) — summary line is "0 applied, N pending" ✓
  • "rolled back" — emitted as "✓ rolled back {migration}" ✓
  • stderr.contains("--yes") || .contains("TTY") — the anyhow::bail! message is "stdin is not a TTY — pass --yes to confirm rollback without a prompt" — hits both branches ✓
• Non-zero exit on missing TTY: anyhow::bail! propagates through main() -> Result<()> which prints to stderr and exits non-zero. ✓
• Isolation: each test creates its own TempDir — no shared state between runs. ✓
• Scope: single new file, no changes to production code. ✓

Non-blocking suggestions

1. test_status_on_empty_database — the DB file does not exist before the call; the test succeeds because migration_status_for_path (via SeaORM) creates the file on first connection. Worth a one-line comment documenting this implicit side-effect so the intent is clear to future readers.

2. Missing --all coverage: migrate down --all --yes is not exercised. A test verifying that all N migrations end up pending after --all would complete the matrix. Non-blocking for this PR.

3. Idempotency: running migrate up twice and asserting the second run still exits zero with "0 pending" would guard against non-idempotent migration regressions. Nice-to-have.