Add shared mutation core preview pipeline

[deverman]

Summary

Add the shared mutation core that later task/project write tools can build on without bespoke transport code.

Change

• add shared typed mutation request/response models in OmniFocusCore
• extend OmniFocusService with a common performMutation entry point
• add preview-only mutation transport support in both bridge and JXA backends
• add cache invalidation hooks to CatalogCache
• invalidate cached project/tag catalog pages after successful non-preview mutations
• add model validation/round-trip tests and cache invalidation tests

Scope Notes

This PR intentionally stops at plumbing:

• no public write MCP tools yet
• no field-specific task/project mutation execution yet
• preview-only mutation flow validates IDs and shared request shape

Validation

• swift test

Why

Issue #29 is about creating one shared mutation pipeline for CLI and MCP before the field-specific write issues begin. This PR gives later issues a stable transport and result envelope without mixing in task/project write behavior.

[deverman]

Merge-order note: PR #40 should merge before issue #30 / the future update_tasks PR. #30 is intentionally stacked on this shared mutation core so task updates reuse the common mutation models, preview/verify flow, and cache invalidation path.

[deverman]

Architecture/stack update: this PR is now retargeted to issue-28-write-contract so the implementation stack is mechanically ordered behind PR #38.

Current intended order: #38 -> #40 -> #41 -> #42 -> #43 -> #44 -> #46.

This keeps the write contract first, then the shared mutation core, then individual mutation tools.

[deverman]

Updated #40 on top of current master after #37 and #38 merged, and retargeted the PR base to master.

Current focused diff: shared mutation request/response models, bridge mutation request plumbing, cache invalidation hooks, and tests.

Validation:

• node --check Plugin/FocusRelayBridge.omnijs/Resources/BridgeLibrary.js
• swift test (73 tests passed; existing MCP SDK deprecation warnings only)
• ./scripts/install-plugin.sh; fully restarted OmniFocus
• ./.build/debug/focusrelay bridge-health-check
• swift run focusrelay benchmark-gate-check --tool task-counts
• swift run focusrelay benchmark-gate-check --tool list-tasks
• swift run focusrelay benchmark-gate-check --tool project-counts
• git diff --check

Merge order note: this should merge after #38 and before #41.