fix: harden plugin lifecycle convergence

[kaitranntt]

Summary

• preserve Normal install as the default while honoring explicit persisted plugin opt-in
• make Claude and Codex plugin install, repair, migration, doctor, and uninstall flows transactional and retry-safe
• project Engineer skill names for Normal installs without changing plugin payload namespaces
• isolate provider tests and add full lifecycle coverage for fresh install, update, repair, uninstall, rollback, and idempotency

Validation

• bun run ci:local (5,269 passed, 65 optional skips, 0 failed)
• UI Vitest: 153 passed
• real Claude/Codex provider canaries in sandbox: 7 passed; host plugin state unchanged
• final Stage 2 code review: no blockers

Docs impact

Major. CLI contract docs updated here; corresponding public documentation PR follows.

Closes #913
Closes #914
Closes #915
Closes #916
Closes #917
Closes #918
Closes #919
Closes #920