Decide (post-v5): fix/undo/sync command model and exit 0 when there is nothing to undo #1352

[agent] Split out of #1088. The maintainer's 2026-10-09 triage narrowed #1088's v5 scope to "a bare scan/get keeps the project's mode" (PR #1317). The other two questions from #1088 are not release requirements, so they are tracked here, without v5-blocker.

Q1, option 3: new command model (next major)

The old verbs would stay as aliases for one major. This is blocked by the C11/C12 engine split (#843, #894). See #1088 for the full rationale (register C34).

Q2: "nothing to undo" exits differently per command (register C74)

On a project with no patch state (a fresh project, or one that a full rollback just emptied), list, apply and vendor --revert exit 0, but rollback and repair exit 1 (manifest_not_found, and rollback uses the legacy {"status":"error","error":"Manifest not found"} shape). So a second rollback exits 1. #1088 has the per-command table and repro.

The proposal (#1088 option A):

• rollback on an empty project returns status: success with zero counts.
• repair on an empty project matches apply (noManifest, exit 0).
• remove <target> with no match keeps exit 1 (not_found).

Tests that pin today's exit 1: in_process_rollback_hosted.rs, mode_migration_vlt.rs, e2e_redirect_vlt_build.rs (L1185-1210) and the covgap rollback/repair suites.

Both parts need a maintainer decision before implementation.