feat(lsp): code actions for missing link diagnostics

[avrabe]

Summary

• Advertise CodeActionProvider capability in LSP server
• Handle textDocument/codeAction requests for "requires at least" / "requires exactly" diagnostics
• Generate QuickFix code actions that insert TODO comments for missing links
• Extracts link type from diagnostic message, places edit on the line after the diagnostic

Implements: REQ-007
Refs: FEAT-010

🤖 Generated with Claude Code

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Rivet Criterion Benchmarks'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: 6ceab41	Previous: f958a7e	Ratio
link_graph_build/10000	34805822 ns/iter (± 1994665)	26664874 ns/iter (± 3447466)	1.31
validate/10000	14038768 ns/iter (± 1007709)	10528470 ns/iter (± 743244)	1.33

This comment was automatically generated by workflow using github-action-benchmark.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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