feat: Add search commit tool

[Not-Dhananjay-Mishra]

Summary

Added a new search_commits tool to the GitHub MCP server, allowing users and LLMs to search for commits across repositories using specialized GitHub search syntax

Why

This provides a missing capability for high-impact commit discovery and debugging that isn't possible with the existing list_commits tool
Fixes #2283

What changed

• Implemented SearchCommits
• Added the tool handler and MinimalSearchCommitsResult struct
• Registered the new tool in pkg/github/tools.go

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

Prompts tested (tool changes only)

• Search for all commits in the 'github/github-mcp-server' repository that mention 'vulnerability' or 'fix' in the last 6 months
• Find all commits authored by 'github' across the organization that were committed after 2024-01-01.
• Find the most recent commit that modified the search.go file with a message containing 'refactor'.
• Search for commits in 'owner/repo' that are tagged with '[RELEASE]' to summarize recent major changes.

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

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

Note: if you're renaming tools, you must add the tool aliases. For more information on how to do so, please refer to the official docs.

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed
• Updated (README / docs / examples)

[Not-Dhananjay-Mishra]

Here are some example use case :