feat(cli): add Go REST CLI and expand v1 API contract

Summary

Screenote can now be driven from a shell or CI script without an MCP client. go install github.com/ivankuznetsov/screenote/cmd/screenote@latest installs a Cobra-based CLI that talks to the REST API, emits JSON on stdout, and reports machine-readable errors on stderr with stable exit codes. MCP remains the rich agent protocol; the CLI is the scriptable REST counterpart, and both share the same query and serialization behavior.

Test plan

• bin/rails test test/controllers/api/v1 test/controllers/api/screenshot_uploads_controller_test.rb — pass
• bin/rails test test/tools — pass (MCP behavior unregressed)
• go test ./... — pass
• Cross-compile smoke for Linux amd64 and macOS arm64 — pass
• go install ./cmd/screenote into a scratch GOBIN — pass

Review summary

Two automated review passes ran against the diff. No High or Medium findings remain open. Fixed items include aggregate annotation list paging/total, error propagation, serializer N+1s, usage exit codes, stable validation JSON, and content-type derivation; tests added for the aggregate path and content-type derivation. One open product decision (screenote config prints the resolved API key; future move to OAuth CLI auth) is a deferred follow-up outside this v1 change.

Linked task

• Slug: add-a-go-cli-for-260708-edec