fix: CLI read-surface — friendly first-run guidance + honor search [path] scope

[srobinson]

Summary

• Found by the live MCP user test against a real corpus.
• Finding C: fresh and empty homes caused stats, links, backlinks, duplicates, and embeddings to expose a raw GenerationReadError. A shared withCurrentGenerationGuidance helper now maps a missing generation to friendly No index found. Run: mdm index … guidance. Source only tree and context remain independent of the index.
• Finding B: the documented positional path in mdm search <query> [path] was a silent no op. An explicit path now scopes keyword, hybrid, and semantic search to that subtree through the existing pathPattern seam, with friendly empty and unindexed path guidance. An omitted path searches the whole corpus and remains independent of the current working directory.
• Personal ~/.mdx examples in the federated knowledge layer design spec are now generic. Product home references to ~/.mdm remain.

Review

• Cross family MoE review: Grok clean. Opus found the omitted path current directory regression, duplicate JSON shape, and glob metacharacter handling. All three were fixed and the delta review was clean.

Test plan

• Focused CLI and path regressions: 75 passed, 0 failed
• Full suite: 1,711 passed, 11 skipped, 0 failed
• Repository lint
• Type checking
• Production build
• Diff checks