Add expectedHeadSha pinning to merge_pull_request

[jcosta1970]

Summary

Adds optional expectedHeadSha pinning to merge_pull_request.

The value is forwarded to github.PullRequestOptions.SHA, which
go-github serializes as the GitHub REST merge endpoint's optional
sha field.

This closes the TOCTOU gap between reviewing a pull request HEAD and merging
it: if the HEAD changes, GitHub rejects the merge instead of merging the new
commit.

Changes

• add optional expectedHeadSha to the merge_pull_request schema;
• pass it to PullRequestOptions.SHA;
• verify the value reaches the REST request as sha;
• verify a 409 head mismatch fails closed with no retry;
• update the merge tool schema snapshot;
• regenerate project docs when local Go tooling is available.

Backward compatibility

The new field is optional. Existing callers that omit
expectedHeadSha retain the current behavior.

Local validation

• Go tests: SKIPPED_GO_NOT_INSTALLED
• generated docs: SKIPPED_GO_NOT_INSTALLED
• golangci-lint: SKIPPED_GOLANGCI_LINT_NOT_INSTALLED
• git diff --check: PASS

Fixes #3181

[CAOShurong]

Reviewed exact head ef6e104f1373ae12a9529bbc8ed4d8041f643fd1.

The optional input is passed directly to go-github's PullRequestOptions.SHA, which serializes it as the REST merge endpoint's sha guard. The focused tests cover both the successful request body and the important failure path: a 409 head mismatch returns an MCP error after exactly one request, so the handler does not silently retry against a different HEAD. Omitting the field keeps the existing behavior.

Verification on this exact head:

• go test ./pkg/github -run '^Test_MergePullRequestSHAPinning$' -count=1 — pass
• go vet ./pkg/github — pass
• go build ./... — pass
• git diff --check — pass

The full go test ./pkg/github -count=1 run still reports the Windows toolsnap icon-order diffs; the same failures reproduce on exact base febc3293a4feb70e62399f39a26b082f78b9b176, so they are not introduced by this patch.

I did not find a blocking issue in the SHA-pinning change.

AI assistance disclosure: Codex assisted with the local verification and base comparison; I verified the cited heads, diff, and results.