refactor: sqlc vs raw SQL — hybrid approach (R3)

Deleted 8 genuinely unused sqlc queries (no inline equivalent):
- UpsertCurrentRelationship, ListEntitiesCapped, ListEntityStatus,
  UpdateSignalState, InsertClassification, InsertFeedback, InsertSkill,
  UpsertCurrentRelationship — all had zero call sites.

Migrated 9 inline raw SQL sites to use sqlc queries:
- GetOntology (impl.go): ListEntityTypes, ListRelationshipTypes,
  ListLifecycleDefs — replaces 3 raw pool.Query blocks with typed sqlcgen
  calls, eliminating manual row scanning.
- EndRelationship (phase3.go): EndCurrentRelationship — replaces tx.Exec
  with sqlcgen.New(tx).EndCurrentRelationship.
- checkPrecondition (impl.go): GetEntityStatus — replaces tx.QueryRow +
  manual Scan with sqlcgen.New(tx).GetEntityStatus.
- GetEntityRelations (impl.go): ListEntityRelations — replaces raw pool.Query
  + scanRelationships helper (now deleted).
- GetGraph (impl.go): ListGraphEdges — replaces raw pool.Query +
  scanRelationships.
- resolveEntityID (impl.go): GetEntityBySlug/GetEntityByID — replaces
  raw pool.QueryRow + Scan.
- createApproval (mcp/server.go): InsertApproval — replaces raw pool.Exec
  with sqlcgen.InsertApproval.

Deleted scanRelationships helper (was only used by the two migrated
graph queries above).

Regenerated sqlcgen — also picks up stale model updates (AgentSession,
SessionPlanStep, SessionQuestion, etc. from recent migrations).

Documented the carve-out in .agents/dev/CONTRIBUTING.md §SQL conventions:
sqlc is the default; raw pool.Query/Exec is reserved for LISTEN/NOTIFY,
dynamic WHERE builders, blast_radius(), and COPY.

go vet, build, httpapi/mcp/db tests all pass. -383/+170 lines.

internal/db/queries/relationships.sql

@@ -22,12 +22,6 @@ WHERE r.valid_to IS NULL
   AND (sqlc.narg('rel_types')::text[] IS NULL OR r.type = ANY(sqlc.narg('rel_types')::text[]))
 ORDER BY r.type, se.slug, te.slug;
 
--- name: UpsertCurrentRelationship :exec
-INSERT INTO relationships (source_id, target_id, type, attributes, valid_from, valid_to)
-VALUES ($1, $2, $3, $4, now(), NULL)
-ON CONFLICT (source_id, target_id, type) WHERE valid_to IS NULL
-DO UPDATE SET attributes = EXCLUDED.attributes;
-
 -- name: EndCurrentRelationship :execrows
 UPDATE relationships SET valid_to = now()
 WHERE source_id = $1 AND target_id = $2 AND type = $3 AND valid_to IS NULL;