backport: issue_graph tool — cross-repo issue/PR hierarchy traversal

[gastrodon]

Summary

Backport the issue_graph tool from upstream PR github#1511 (tracking issue: github#1510).

For autonomous agents this collapses N+1 issue traversal calls into one, returning a complete graph of epics, tasks, sub-issues, and linked PRs.

What the tool does

BFS traversal from a root issue/PR following:

• Sub-issues (native GitHub sub-issue relationship)
• Linked PRs (closing references)
• Parent epics (tracked-in)

Returns a flat node+edge list optimized for LLM consumption.

Parameters:

• owner (required)
• repo (required)
• issue_number (required): int
• focus (optional): "epic" or "batch" — filters traversal direction
• cross_repo (optional, bool): follow references into other repos

Backport approach

1. Fetch the upstream diff: curl https://github.com/github/github-mcp-server/pull/1511.diff
2. Read it carefully — the PR was generated by Copilot against an older main, expect drift
3. Implement the tool manually in pkg/github/issues.go or a new pkg/github/issue_graph.go
4. Register in the issues toolset

Key implementation notes

• Sub-issues use the subIssues GraphQL connection — requires sub-issues beta header. Find where beta headers are added in the codebase (look for sub-issues or betas.go)
• The BFS traversal should have a depth/node limit to prevent runaway queries (upstream uses 50 nodes)
• Cross-repo references need careful handling — only follow if cross_repo=true
• Output format: flat array of {id, type, title, state, url, relationships: [{type, target_id}]}

Reference

Upstream PR diff: https://github.com/github/github-mcp-server/pull/1511.diff
Upstream issue: github#1510

Files to Modify

• pkg/github/issues.go or new pkg/github/issue_graph.go
• pkg/github/issues_test.go or new pkg/github/issue_graph_test.go
• toolset registration for issues toolset