feat(repositories): add delete_branch tool

[huzaifa678]

Description

Adds a delete_branch tool to the repositories toolset, resolving #1476.

The repo already exposes create_branch and list_branches but had no way to delete a branch. This fills that gap, following the existing create_branch implementation pattern.

Guardrail

As requested in the issue, the tool will not delete a protected branch. It fetches the branch first and, if protected is true, returns a tool error rather than attempting the deletion — giving the model actionable feedback.

Behavior

Aspect	Value
Tool name	delete_branch
Required params	owner, repo, branch
ReadOnlyHint	false
Scope	repo

Flow:

1. GET /repos/{owner}/{repo}/branches/{branch} → check protection status
2. If protected → return error, do not delete
3. Otherwise DELETE /repos/{owner}/{repo}/git/refs/heads/{branch}

Testing

Test_DeleteBranch covers:

• successful deletion of an unprotected branch
• refusal to delete a protected branch
• branch not found
• deletion API failure

Verified locally:

• go test ./... — pass
• golangci-lint run — 0 issues
• script/generate-docs — README updated
• UPDATE_TOOLSNAPS=true go test ./... — snapshot committed

Checklist

• Tests added
• script/lint passes
• Toolsnap committed (pkg/github/__toolsnaps__/delete_branch.snap)
• README regenerated via script/generate-docs
• Change is focused on a single tool

Closes #1476