🏷️ Search tag routing (tagForUrl)

PR fix/assess-followups — type a docs URL and see which content-type tab the unified ⌘K index files it under. This is the exact branch logic the new search-indexes.test.ts now covers.

Try a URL

Routing rules (first match wins)

URL prefix→ tag
/docs/reference/skills · /docs/skillsskill
/docs/reference/agents · /docs/agentsagent
/docs/reference/hooks · /docs/hookshook
everything elsedocs
data-only compositions (injected)composition
The tab you pick in the ⌘K dialog sets ?tag=, and the server filters the one Orama index by this field. Previously these were untested; this PR adds unit coverage for every branch + the buildSearchIndexes enrichment + the agent taskTypes filter.