Add patch CLI command for existing memory documents

[G9Pedro]

Summary

• add a new patch command implementation at src/commands/patch.ts
• support three patch modes:
  • clawvault patch <path> --append "content"
  • clawvault patch <path> --replace "old" --with "new"
  • clawvault patch <path> --section "## Name" --content "body" (section upsert)
• preserve YAML frontmatter by delegating document mutation to ClawVault.patch(...)
• keep indexing scoped to the patched document via existing vault patch/reindex behavior
• register registerPatchCommand in src/cli/index.ts
• add focused tests in src/commands/patch.test.ts for all modes and CLI registration

Testing

• npm run build && npx vitest run src/commands/patch.test.ts
• npm test

Notes

• followed scope limits: only added new src/commands/patch.ts and src/commands/patch.test.ts, and updated only the CLI entry point registration file (src/cli/index.ts).