Two new tag kinds on the derived registry — skill:<name> and agent:<name> —
mined from what conversations actually did, next to the project/service/stack
axis that says where they worked.
Backend: conversation cards and the detail now carry skillsUsed (the Skill()
calls, a conversation's subagents' included — the indexer keeps that map in the
transcripts table's own column, so main._skills_by_path joins it back) and
agentsUsed (built from the same run list the agents catalog counts, so a card
can never disagree with the agent page).
Frontend: useTags() folds both catalogs in — everything on disk plus the
built-ins that actually ran — with per-tag conversation counts and last-used;
SkillTags/AgentTags render on the conversation detail (capped at 5, expanding
+N); the Skills page takes ?skill=<name> so a skill pill lands on one card.
Skills and agents are composer chips too (toolMentions/toolTags): picking one
adds a 'Use the following in this task:' bullet naming the skill/subagent —
deliberately not part of the Work-in-location block.
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