feat: loadout rm — artifact removal with clean uninstall

[machado144]

What

Spec 009 (spec · plan, 10/10 steps done): the store's missing delete verb.

`loadout rm ...` uninstalls from every agent home and deletes from the store in one confirmation-gated pass — files deleted, managed blocks stripped, and (new) merged JSON entries removed surgically. Flags: `--dry-run`, `--yes`, `--force`, `--keep-store`.

Why

• No way to remove an artifact except hand-editing `loadout.yaml`.
• ROADMAP "Open questions": removing an MCP/hook artifact leaked its merged `settings.json` / `.claude.json` entry forever. Closed here via `jsonRemove`, the exact inverse of `jsonMerge` (delete fragment keys, deep-equal array-item removal, prune emptied objects, preserve every other byte).

Example

$ loadout rm demo lint gh --yes
  delete claude-code: skills/demo  (demo)
  delete claude-code: hooks/loadout-lint.sh  (lint)
  update claude-code: settings.json  (lint)
  update claude-code: .claude.json  (gh)
  delete store: skills/demo
  delete store: hooks/lint
  delete store: mcps/gh
Removed: 4 agent change(s), 3 store artifact(s).

User-authored keys keep their exact bytes; `mcpServers`/`hooks` objects are pruned only when the removal empties them.

Safety

• Confirmation gate (invariant suite extended to `rm`), pre-write snapshot with trigger `rm` that also captures the store manifest + content under the same `store` pseudo-agent spec 008 introduced — `loadout restore` resurrects a regretted rm even in a non-git store.
• Drift-safe: targets you edited since install are skipped with a warning; `--force` removes them (snapshot first). Skipped targets keep their install records so a later `--force` still works.

Behavior change

`loadouts switch` now skips drifted targets instead of deleting them (strictly safer; `--force` restores the old behavior). One integration test updated to the new semantics.

Testing

TDD: failing engine tests reproducing the JSON leak came first. 10 `jsonRemove` behavior tests, drift/retain/ApplyRemoval engine tests, 3 `Store.RemoveArtifact` tests, 7 integration scenarios driving the built binary (full removal, declined confirm, keep-store, force/drift, unknown ID, source-origin warning, restore round-trip). Rebased on #20 (spec renumbered 008→009, store snapshots harmonized onto `SnapshotAgent`); `make ci` + `make test-integration` green on the rebased tree.

🤖 Generated with Claude Code

[github-actions]

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Current title: feat: loadout rm — artifact removal with clean uninstall