fix(target): make --target flag case insensitive to match Tanka

[julienduchesne]

Summary

• Tanka's --target flag performs case-insensitive regex matching on kind/name, but rtk was using case-sensitive matching
• Fixed both target filtering code paths to use RegexBuilder::new().case_insensitive(true):
  • extract_manifests() in util.rs (used by show, apply, diff, prune)
  • compile_target_patterns() in export.rs (used by export)
• Added golden test target_filter_case_insensitive_env that uses --target "configmap/.*" (lowercase) to match ConfigMap resources

Test plan

• Added golden test case reproducing the issue (configmap/.* filter against ConfigMap resources)
• Verified test fails before fix (rtk exports nothing, tk exports the ConfigMap)
• Verified all 33 golden fixture tests pass after fix
• Verified all rtk unit tests pass

🤖 Generated with Claude Code

[github-actions]

Benchmark Results

Benchmark	Test	vs tk	vs base
Diff	cluster_scoped	53.76x faster	equal
Diff	configmap_unchanged	29.18x faster	equal
Diff	deployment_nested_changes	28.09x faster	equal
Env List	Single Inline Directory (--json)	46.17x faster	equal
Env List	Single Inline File (--json)	45.56x faster	equal
Env List	All Environments (--json)	88.0x faster	equal
Eval	Single Static Environment	14.56x faster	equal
Eval	Inline Environment File	22.67x faster	equal
Eval	Eval Expression - Deeply Nested Path	15.63x faster	equal
Export (Full)	Single Static Environment	5.56x faster	equal
Export (Full)	Single Inline Environment	14.5x faster	equal
Export (Full)	All Environments	11.16x faster	equal
Export (Replace)	Single Static Environment	5.22x faster	equal
Export (Replace)	Single Inline Environment	14.97x faster	equal
Export (Replace)	All Environments	19.81x faster	equal
Tool Importers Count	Single Lib Directory (Non-Recursive)	2.17x faster	equal
Tool Importers Count	Single Lib Directory (Recursive)	2.19x faster	equal
Tool Importers Count	Entire Lib Directory (Non-Recursive)	1.47x faster	equal
Tool Importers Count	Entire Lib Directory (Recursive)	1.16x faster	equal
Tool Importers	Global Lib File	2.79x faster	equal
Tool Importers	Single Env-Specific Lib File	1.68x faster	equal
Tool Importers	All Env-Specific Lib Files	6.71x faster	equal
Tool Importers	All Lib Files	6.82x faster	equal
Tool Importers	All Jsonnet Files	11.79x faster	equal
Tool Imports	Single Static Environment	3.54x faster	equal
Tool Imports	Single Inline Environment File	3.59x faster	equal

Full results available in workflow artifacts.

Benchmark run on commit b9974f18a34b04090259673cb8b88c2640a7d4c6