feat: add batch pull request review threads tool

[rodboev]

Summary

Adds get_pull_request_review_threads_batch, a read-only MCP tool for fetching review threads for an explicit list of pull requests in one batch contract. This replaces the current N-call workflow where callers must invoke pull_request_read with get_review_comments once per PR and manage per-PR errors and cursors themselves.

Why

The current review-thread read path exists only inside pull_request_read, where method=get_review_comments hydrates one PR at a time through GraphQL. That single-PR implementation already returns the right structured thread payload, but there is no batch contract for explicit PR lists and no higher-level way to carry per-PR cursors across multiple reads.

Fixes #2301

What changed

• pkg/github/pullrequests_batch_review_threads.go: adds the new batch review-thread tool, per-PR cursor handling through afterByPullNumber, duplicate-input deduplication, and partial-success response handling for per-PR GitHub/GraphQL failures.
• pkg/github/tools.go: registers the new batch review-thread tool.
• pkg/github/pullrequests_batch_review_threads_test.go: covers schema, multi-PR success, cursor forwarding, partial GraphQL failures, duplicate-input deduplication, and cap validation.
• pkg/github/__toolsnaps__/get_pull_request_review_threads_batch.snap: added for the new tool schema.
• README.md: regenerated tool documentation.

MCP impact

• New tool added
  Adds a new read-only batch tool; the existing single-PR review-thread method remains unchanged.

Prompts tested (tool changes only)

• "Fetch review threads for PRs 12, 18, and 25 so I can summarize recurring review feedback."
• "Read review comments for this explicit PR list and keep each PR's thread metadata separate."

Security / limits

• Data exposure, filtering, or token/size limits considered
  The tool stays repo-scoped and read-only; the input cap and shared per-page limit bound GraphQL fan-out and payload size.

Tool renaming

• I am not renaming tools as part of this PR

Lint & tests

• Linted locally with ./script/lint
  Direct lint command passed locally: golangci-lint run reported 0 issues.

• Tested locally with ./script/test
./script/test was not run locally. Focused package and generation checks passed locally:

• go test ./pkg/github -run Test_GetPullRequestReviewThreadsBatch - passed. Covers schema/toolsnap, multi-PR success, per-PR cursor forwarding, partial GraphQL failures, duplicate-input deduplication, and validation errors.

• $env:UPDATE_TOOLSNAPS='true'; go test ./pkg/github; Remove-Item env:UPDATE_TOOLSNAPS - passed. Regenerated the new tool snapshot and re-ran the pkg/github package tests.

• bash script/generate-docs - passed. Regenerated tool docs; the final diff only updates README.md.

Docs

• Updated (README / docs / examples)
  Regenerated via script/generate-docs; toolsnaps were updated in the same implementation run.