feat: support subdirectories in .one/flows/

[moekatib]

Summary

• Flows can now be organized into subdirectory groups: .one/flows/<group>/<key>/flow.json
• flow list shows group-prefixed keys (e.g. research/company-research)
• flow execute and flow validate resolve both namespaced keys (research/company-research) and bare keys (scans subdirs automatically)
• flow create accepts group/key format to place flows in subdirectories
• Fully backward compatible — ungrouped flows and legacy single-file layout work unchanged

Closes #96

Test plan

• flow list shows grouped and ungrouped flows correctly
• flow validate research/company-research resolves namespaced key
• flow validate company-research scans subdirs for bare key
• flow create infra/cache-clear --definition ... saves to correct path
• Ungrouped flows resolve as before
• Build clean, no type errors

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