Plugin-first cleanup: migrate plugin off legacy hook shelling

[G9Pedro]

Summary

• migrated plugin context injection (vault-context-injector.ts) from CLI subprocess calls to in-process library usage (ClawVault.find() + buildSessionRecap()), removing pre-prompt shell execution
• migrated session lifecycle recover/checkpoint handling to in-process command-library calls in session-lifecycle.ts
• kept observer as CLI-based execution but switched plugin observer trigger path to non-blocking spawn instead of synchronous execFileSync
• removed legacy hooks/clawvault/ handler tree and dead packages/plugin/ scripts package
• added .npmignore exclusions for non-publish directories and updated release metadata (package.json, openclaw.plugin.json, CHANGELOG.md) for 3.5.0
• added targeted plugin tests for the new in-process context/lifecycle paths

Validation

• npm run build ✅
• npx vitest run src/plugin/vault-context-injector.test.ts src/plugin/hooks/session-lifecycle.test.ts ✅
• npm test ✅ (120 files, 745 tests passing)
• npm publish --dry-run ✅ (clawvault@3.5.0)

Notes

• no changes were made to memory_search / memory_get tool registration
• plugin config schema and communication protocol behavior were preserved