Refactor bridge mutation helpers

[deverman]

Summary

• Extracts shared bridge mutation helpers for primary-key indexing, per-item result construction, response envelopes, verified message formatting, and save-if-mutated behavior.
• Reuses the shared ID indexer for task, project, tag, and folder lookup paths.
• Keeps CLI and MCP request/response shapes unchanged while reducing repeated mutation boilerplate.

Stack / merge order

Depends on PR #50. Merge order remains:

1. Improve bridge timeout diagnostics #49
2. Implement move_projects folder moves #50
3. this PR
4. Document AI-Optimized Mutation Workflows for CLI and MCP #36 docs / AI guidance

Validation

• node --check Plugin/FocusRelayBridge.omnijs/Resources/BridgeLibrary.js
• swift test
• ./scripts/install-plugin.sh
• Restarted OmniFocus
• ./.build/debug/focusrelay bridge-health-check
• Live temp-project smoke: preview move to Travel, move to Travel with verify, move back to root with verify, delete temp project and confirm deletion

Refs #48

[deverman]

Stack note: this PR is intentionally based on #50. Merge #49 first, then #50, then #51. After #51, continue with #36 docs / AI guidance.

[deverman]

Stack note update: #52 (issue #36 AI-optimized mutation docs) is now stacked on this PR. Merge #49, #50, #51, then #52.