Add issue dependency tools and issue_read get_parent

[zwick]

Summary

Add MCP support for issue relationships not previously reachable: read/write of an issue's blocked-by / blocking dependencies, plus reading an issue's parent. The dependency tools are gated behind a feature flag; the parent read is a method on the existing issue_read tool.

Why

github-mcp-server could read an issue's children (issue_read → get_sub_issues) but had no way to read its parent, and no way to read or edit dependencies at all. The GitHub API supports both, and the gh CLI shipped the dependency verbs in v2.94.0; this closes the MCP gap.

Refs #2391, #950

What changed

• issue_read → new get_parent method — reads the GraphQL Issue.parent (upward counterpart to get_sub_issues); returns a null parent when the issue isn't a sub-issue. Always-on, mirroring get_sub_issues.
• issue_dependency_read — get_blocked_by / get_blocking via the Issue.blockedBy / Issue.blocking connections, cursor-paginated.
• issue_dependency_write — add / remove × blocked_by / blocking via the addBlockedBy / removeBlockedBy mutations. Accepts issue numbers (resolved to node IDs in one aliased query); blocking is the inverse of blocked_by with subject/related swapped.
• Both dependency tools gated behind a new issue_dependencies feature flag — an opt-in flag (user-enableable via --features / X-MCP-Features, and auto-enabled in insiders mode), kept out of the default toolset. Mirrors the file_blame precedent (present in both AllowedFeatureFlags and InsidersFeatureFlags).

MCP impact

• Tool schema or behavior changed — issue_read gains a get_parent method (negligible schema cost).
• New tool added — two flag-gated dependency tools. Added fixed-context cost to the default toolset is 0 (gated); combined schema is ~700–900 tokens when the flag is enabled.

Prompts tested (tool changes only)

• "What's the parent of issue Issue: Fix incorrect VS Code installation URL (both buttons point to Insiders) #123 in owner/repo?" → issue_read / get_parent
• "What issues block issue Issue: Fix incorrect VS Code installation URL (both buttons point to Insiders) #123?" → get_blocked_by
• "What does issue Issue: Fix incorrect VS Code installation URL (both buttons point to Insiders) #123 block?" → get_blocking
• "Mark issue Issue: Fix incorrect VS Code installation URL (both buttons point to Insiders) #123 as blocked by Update VS Code config JSON #45" → add + blocked_by
• "Remove the dependency where Issue: Fix incorrect VS Code installation URL (both buttons point to Insiders) #123 blocks Update VS Code config JSON #45" → remove + blocking

Security / limits

• Auth / permissions considered — requires repo scope; reads are read-only.

Tool renaming

• I am not renaming tools as part of this PR

Lint & tests

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

Docs

• Updated (README for get_parent; feature-flags / insiders docs for the dependency tools)

[Copilot]

Pull request overview

Adds new feature-flagged MCP tools to read and modify GitHub Issue dependency relationships (blocked-by / blocking) using GraphQL connections and the addBlockedBy / removeBlockedBy mutations, filling a gap not covered by existing issue relationship tools.

Changes:

• Introduces issue_dependency_read (GraphQL Issue.blockedBy / Issue.blocking) with cursor pagination.
• Introduces issue_dependency_write (GraphQL addBlockedBy / removeBlockedBy) resolving issue numbers to node IDs in a single aliased query.
• Adds the issue_dependencies feature flag, tests, toolsnap snapshots, and regenerated docs sections.

Show a summary per file

File	Description
pkg/github/tools.go	Registers the new issue dependency tools in the overall tool inventory (feature-flag gated).
pkg/github/issue_dependencies.go	Implements read/write tools and supporting GraphQL query/mutation helpers.
pkg/github/issue_dependencies_test.go	Adds schema snapshot validation and behavior tests for the new tools.
pkg/github/feature_flags.go	Introduces the issue_dependencies flag and wires it into feature-flag sets.
pkg/github/toolsnaps/issue_dependency_read_ff_issue_dependencies.snap	Adds the flag-gated schema snapshot for the read tool.
pkg/github/toolsnaps/issue_dependency_write_ff_issue_dependencies.snap	Adds the flag-gated schema snapshot for the write tool.
docs/insiders-features.md	Documents the new issue_dependencies flag/tools in insiders features (generated section).
docs/feature-flags.md	Documents the new issue_dependencies flag/tools in feature flags (generated section).

Copilot's findings

• Files reviewed: 8/8 changed files
• Comments generated: 1