Epic: Agent plugin and harness install verification follow-through

## Goal
Close the remaining harness-install gaps found during the 2026-06-03 Claude/Codex/Kiro/Antigravity audit.

The immediate Codex runtime fix is in PR #1643. This epic tracks the follow-through work that should not be buried inside that fix PR.

## Child issues

- [x] #1644
- [x] #1645
- [ ] #1646
- [x] #1647

## Scope

- Remove stale active Codex `.codex` install guidance from docs.
- Refresh command renderer snapshots or templates so Codex/Vibe lifecycle wording is canonical again.
- Verify whether Antigravity has a real plugin/package install surface and document the actual targets.
- Verify whether Kiro has any plugin/Powers package surface before adding it to plugin-install scope.

## Non-goals

- Do not duplicate #1635. Claude/Codex plugin installation remains tracked there.
- Do not claim Antigravity or Kiro plugin support until package shape, install locations, and invocation behavior are verified against current upstream tooling.

## Related

- #1635
- #1643