Preflight every bulk mutation target before writing

[deverman]

Closes #144

Acceptance journey

A mixed batch containing one valid task ID and one unknown ID fails the entire request before any apply or save, leaving the valid task unchanged.

Validation impact

mutation

Changes

• preflight all target IDs in the shared Bridge mutation executor
• return per-target failures that explicitly state no targets changed
• document all-or-nothing target resolution in the public tool descriptions and write contract
• add batch and per-item save-mode regression coverage

Validation

• swift test — 161 tests passed
• swift run focusrelay-dev validate --impact mutation — 161 tests and release build passed
• live Bridge UAT after plugin install and OmniFocus restart:
  • edit-tasks j6U8yT2FZht lbJReNIcjBB --operation set_completion --state completed --verify
  • result: 0 successes, 2 failures, both report no targets changed
  • readback: valid task remained completed: false

Note: the AGENTS example includes --base origin/master, but the current focusrelay-dev validate command rejects --base; validation used the supported syntax above.