D67 — concept_delete MCP tool

Context. The control-plane exposed concept_write (create/update), concept_move (move), supersede (mark superseded) but no way to actually remove a concept: deleting a page required direct git on the repo (clone + git rm + push, then the pod realigns on pull), outside the MCP control-plane.

Decision. New concept_delete(id, [if_match]) with the standard write invariants: KB.DeleteConcept rejects empty IDs and reserved files, resolves the path in write-mode (blocks traversal) and os.Remove with ErrNotFound if absent; optional if_match for optimistic concurrency (stale_write). Registered under gitWrap → automatic git commit (CommitOp also stages removals). Updates the derived indexes: search.Index.Remove (exported wrapper of the existing remove) via liveIndex.remove, and sqlindex.Index.Delete (best-effort, log to stderr, like concept_write's upsert). Like concept_move, it does not update incoming backlinks: the return message points to lint. Classified write (no need to touch readonly.go, fail-closed default); added to the agent tool profile (D65) as a core tool.