add(treesitter): extract Ruby callable symbols

[odvcencio]

Summary

• add a Canopy-owned Ruby tags query for modules, classes, instance methods, singleton methods, and call references
• add a focused regression for nested Ruby containers and callable extraction
• restore Ruby callable nodes to graph, dead-code, impact, complexity, hotspot, and structural-context consumers

Addresses the Ruby portion of #3. Kotlin and PHP remain explicitly out of scope.

Evidence

• exact origin/main baseline: the regression reported no module, class, instance-method, or singleton-method definitions
• feature: the same regression passes and indexes six symbols from the representative Ruby fixture
• call graph: resolves total_with_tax -> tax_rate and default_rate -> lookup_rate, with zero unresolved calls
• dead-code analysis: evaluates four Ruby callables instead of having no callable definitions to inspect

Validation

• go test ./pkg/lang/treesitter -run '^TestRubyCallableExtraction$' -count=1 -v
• go test ./pkg/lang/treesitter -count=1
• go test ./pkg/index/... -count=1
• go test -race ./pkg/lang/treesitter -run '^TestRubyCallableExtraction$' -count=1
• go test ./... -count=1
• staged secret scan: no leaks found