feat: issue_graph — cross-repo issue/PR hierarchy traversal

[gastrodon]

Summary

Backport of upstream github#1511. Implements the issue_graph tool that traverses issue/PR hierarchies via BFS and returns a flat node+edge list optimized for LLM consumption.

Closes #3

What this adds

• issue_graph tool registered in the issues toolset (alongside issue_read, sub_issue_write, etc.)
• BFS traversal from a root issue/PR, following:
  • Sub-issues (GitHub sub-issues API)
  • Parent issues (GraphQL parent field, requires no beta header in v87)
  • Body references (closes #N, fixes #N, cross-repo owner/repo#N, full GitHub URLs)
  • Cross-referenced issues from the timeline (focus node only)
• Node classification: epic (by issue type, label, or title), batch (has sub-issues), task, pr
• Priority queue ensures parents are visited before siblings, children before cross-refs
• focus=epic|batch — shifts focus up the hierarchy to the nearest epic or batch
• cross_repo=false (default) — skips edges that point to a different repo than root
• max_nodes=50 (default) — BFS node limit to prevent runaway crawls
• 30-second crawl timeout; returns partial results on timeout

Output format (text, LLM-optimized)

GRAPH SUMMARY
=============
Focus: #42 (task) "Fix login bug"
State: open
Graph contains 1 nodes: 1 task(s)

Node types: epic (large initiative), batch (has sub-issues), task (regular issue), pr (pull request)

NODES (1 total)
===============
#42|task|open|Fix login bug [FOCUS]
  Preview: Users cannot log in after the recent update.

SUB-ISSUES (parent → child)
===========================
(none)

CROSS-REFERENCES (mentioned/referenced)
=======================================
(none)

Test plan

• go build ./... passes
• All new tests pass (TestIssueGraph_*, TestExtractIssueRefs, TestClassifyIssueNode, TestSanitizeBodyPreview)
• Existing TestAllToolsHaveRequiredMetadata, TestAllToolsHaveHandlerFunc, TestAllToolsetIconsExist all pass
• Toolsnap created at pkg/github/__toolsnaps__/issue_graph.snap
• Manual end-to-end test against a real repo with sub-issue hierarchy