Prepare dev release workflow

[leehack]

Summary

• mark prerelease versions as GitHub prereleases and keep them out of latest release selection
• add a publish dry-run gate before pub.dev publish
• document the MCP 2026 RC dev release sequence

Validation

• ruby YAML parse for publish/release workflows
• dart analyze
• dart test test/docs/markdown_docs_test.dart
• dart pub publish --dry-run
• dart run tool/validate_cli_publish.dart

Notes

This PR prepares the dev-release path for mcp_dart 2.3.0-dev.0 and mcp_dart_cli 0.2.0-dev.0. The SDK should be published first, then the CLI should be validated with --published-sdk before its release.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.75%. Comparing base (0c5ca39) to head (6cd000f).
⚠️ Report is 5 commits behind head on dev/2026-07-28-rc.

Additional details and impacted files

@@                Coverage Diff                 @@
##           dev/2026-07-28-rc     #275   +/-   ##
==================================================
  Coverage              84.75%   84.75%           
==================================================
  Files                     70       70           
  Lines                  15364    15364           
==================================================
  Hits                   13022    13022           
  Misses                  2342     2342           

Flag	Coverage Δ
cli	76.73% <ø> (ø)
unittests	88.24% <ø> (ø)

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