feat(pull_requests): add MCP tools for GitHub Stacked PRs (#2905)

[ruhaanpathan]

Summary

Adds 5 new MCP tools (get_stack, list_stacks, link_stack, update_stack, dissolve_stack) to the pull_requests toolset to support native GitHub Stacked PRs.

Why

Agents managing stacked/dependent PRs currently lack a first-class way to discover, create, update, and dissolve native GitHub Stacked PR objects via MCP without needing host gh CLI access.
Fixes #2905

What changed

• Added pkg/github/pullrequests_stacks.go implementing get_stack, list_stacks, link_stack, update_stack, and dissolve_stack.
• Registered all 5 Stacked PR tools in pkg/github/tools.go under ToolsetMetadataPullRequests.
• Updated generatePullRequestsToolsetInstructions in pkg/github/toolset_instructions.go with stacked PR guidance.
• Added unit test suite in pkg/github/pullrequests_stacks_test.go.

MCP impact

• New tool added
  Added get_stack, list_stacks, link_stack, update_stack, and dissolve_stack tools under the pull_requests toolset.

Prompts tested (tool changes only)

• "Show me the pull request stack details for stack chore: remove deprecated func #10" -> triggers get_stack(owner, repo, stackNumber: 10)
• "List all pull request stacks in this repo" -> triggers list_stacks(owner, repo)
• "Link pull requests Add optional string array param #101 and VS Code install steps #102 into a stack based on main" -> triggers link_stack(owner, repo, pullNumbers: [101, 102], base: "main")
• "Update stack chore: remove deprecated func #10 with pull requests Add optional string array param #101, VS Code install steps #102, Fix VS Code inputs link #103" -> triggers update_stack(owner, repo, stackNumber: 10, pullNumbers: [101, 102, 103])
• "Dissolve stack chore: remove deprecated func #10" -> triggers dissolve_stack(owner, repo, stackNumber: 10)

Security / limits

• Auth / permissions considered
  Requires standard repo OAuth scope matching other pull request management tools.

Tool renaming

• I am renaming tools as part of this PR (e.g. a part of a consolidation effort)
  • I have added the new tool aliases in deprecated_tool_aliases.go
• I am not renaming tools as part of this PR

Lint & tests

• Tested locally with ./script/test
  Added unit tests covering tool definitions, input schemas, and HTTP API execution handlers.

Docs

• Updated (README / docs / examples)
  Updated toolset instructions for pull_requests.

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