feat: add create_branch tool to repos toolset

[wuqunfei]

Summary

• Adds a create_branch write tool to the repos toolset, wrapping octokit's git.createRef. Resolves the from input (a branch name or commit SHA) into a target SHA by trying a branch lookup first and falling back to treating it as a raw SHA on a 404, then creates the new branch at refs/heads/<branch>.
• Non-404 errors from the branch-lookup step propagate immediately rather than being silently reinterpreted as "must be a SHA."
• Bumps tool-count references (104 → 105) and write-tool-count references (28 → 29) across README.md, package.json, and manifest.json.
• Stops tracking docs/ (internal design specs/plans), matching the existing .husky/ precedent.

Test plan

• npm test — 224/224 tests passing (23 files), including 6 new tests for create_branch: happy-path branch resolution, SHA fallback on 404, non-404 error propagation, downstream createRef failure, and read-only/read-write permission gating
• npm run typecheck — clean
• npm run lint — clean
• Verified via buildServer that read-only registers 76 tools, read-write registers 105 tools, with zero duplicate tool names
• Went through full design spec → implementation plan → subagent-driven execution with per-task review and a final whole-branch review; two doc-accuracy gaps (manifest.json stale "104", README's stale "28 mutating tools") were caught by the final review and fixed in a follow-up commit