Document AI-optimized mutation workflows

[deverman]

Summary

Closes #36.

Adds user-facing docs for the shipped v1 mutation surface:

• new docs/mutation-workflows.md with CLI and MCP routing guidance
• README examples and mutation tool overview
• docs/use-cases.md status refresh so shipped write tools are no longer described as future
• write contract link to the workflow companion doc

Notes

The docs emphasize the locked v1 constraints:

• ID-only mutation targeting
• homogeneous bulk writes only
• patch vs lifecycle/status vs move tool split
• previewOnly, verify, and compact returnFields
• explicit out-of-scope items including mixed batch_mutate_tasks, create/delete tools, planned-date writes, and folder discovery

Validation

• git diff --check
• swift test passed: 95 Swift Testing tests