feat(go): Add Go stdlib extraction tool for registry generation (PR-21)

[shivasurya]

Summary

• Add tools/internal/goextract package implementing Extractor for walking $GOROOT/src and extracting the full exported API surface of Go stdlib packages via go/parser and go/ast
• Extract functions (signatures, params, returns, generics, variadic, methods), types (struct/interface/alias with fields and methods), constants (iota detection), and package-level variables with docstrings and deprecation markers
• Generate per-package JSON files ({pkg}_stdlib.json) and manifest.json with SHA256 checksums and aggregate statistics (186 packages from Go 1.26 stdlib)
• Add CLI entry point tools/generate_go_stdlib_registry.go with --go-version, --output-dir, and --goroot flags
• Achieve 100% test coverage with 40+ unit tests and integration tests against real Go 1.26 stdlib

Test plan

• go build ./... passes with zero errors
• golangci-lint run ./... reports 0 issues
• go test ./tools/internal/goextract/ — 100% coverage, all tests pass
• Integration test extracts 186 packages from Go 1.26 stdlib, verifies fmt.Println variadic, slices.Sort generic, os.Stdin variable
• Full module test suite go test ./... — all 29 packages pass
• JSON output uses snake_case keys (import_path, is_variadic, is_generic, etc.) matching core types from PR-01

🤖 Generated with Claude Code

[safedep]

SafeDep Report Summary

No dependency changes detected. Nothing to scan.

_{This report is generated by SafeDep Github App}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.14%. Comparing base (5f008bf) to head (d0b0db7).

Additional details and impacted files

@@                       Coverage Diff                        @@
##           shiva/go-stdlib-support-pr01     #548      +/-   ##
================================================================
+ Coverage                         82.68%   83.14%   +0.45%     
================================================================
  Files                               140      141       +1     
  Lines                             17143    17610     +467     
================================================================
+ Hits                              14174    14641     +467     
  Misses                             2427     2427              
  Partials                            542      542              

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