v0.6.2

Breaking: learn response changed from flat {kbid, indexed_terms_count}
to standardized list payload {items, total, offset, superseded,
warnings, near_duplicates} across all layers.

New: skill/agent search, content --search, enriched status,
--format mcp envelope, MCP tool text readability.

Fixes: search --content formats, --format text fallback,
concurrent write crash guard.

Docs: MCP quick start, installation guides, breaking changes
in release notes.