Define v1 write contract and mutation checklist

[deverman]

Summary

Define the v1 write contract and mutation change checklist before implementing write tools.

Change

• add docs/omni-automation-write-contract.md
• add docs/mutation-change-checklist.md
• lock the v1 mutation surface for tasks and projects
• document homogeneous bulk, ID-only targeting, preview/verify, cache invalidation, and out-of-scope areas

Why

Issue #28 is the write-side equivalent of the existing query contract. It removes API-shape ambiguity before mutation work starts and keeps future implementation aligned with documented Omni Automation APIs.

Validation

• swift test

Notes

swift test is currently blocked by the existing swift-sdk compiler failure in .build/checkouts/swift-sdk/Sources/MCP/Base/Transports/NetworkTransport.swift on this machine/toolchain. This PR changes docs only.

[deverman]

Merge-order note: this PR is intentionally docs-only and should merge after PR #39. PR #39 updates swift-sdk to 0.12.0 so swift test passes again on Swift 6.3.1 / Xcode 26.4.1. Keep this PR scoped to the write-contract docs; do not fold the dependency update into it.

[deverman]

Dependency follow-up: PR #39 has merged, this branch has been updated from origin/master, and swift test now passes cleanly on Swift 6.3.1 / Xcode 26.4.1. The validation blocker mentioned in the original PR body is no longer present.