feat(capabilities): metadata, frozen registry, scoped grants and requests

- Capabilities declare access (read/write, unknown fails closed as
  write), side effects, idempotency, timeout, cost estimate and the trust
  of their output (web content is low trust). A capability may derive its
  policy target from its parameters; a disagreeing target is refused, and
  approvals whose modified parameters change the target are refused.
- The registry is frozen at start-up; registration afterwards panics.
- Invocation.RequireApproval turns ALLOW into ASK for commitments; it can
  only tighten, and scoped grants never satisfy it.
- Scoped grants (migration 0025): capability, target and parameter globs,
  access, expiry of at most seven days and an optional operation count
  consumed atomically; applied only where policy says ASK. A sweep marks
  lapsed grants and refreshes the self-model, which now lists live grants.
- Capability requests carry scope, duration, operation cap, reason,
  evidence, risk, mitigation, fallback, alternatives and explicit decision
  history; they batch per intention, are capped per mind (operator
  attention budget) and can be decided only by a named person. Component
  names are refused as deciders, for approvals too, and as usernames.
- API routes, OpenAPI entries, scheduler sweeps and approval-centre
  sections for requests and temporary grants.

internal/auth/store.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/jackc/pgx/v5/pgxpool"
 
+	"github.com/andydixon/noema/internal/security/actors"
 	"github.com/andydixon/noema/internal/security/secrets"
 	"github.com/andydixon/noema/internal/storage"
 	"github.com/andydixon/noema/pkg/ids"
@@ -79,6 +80,9 @@ func (s *Service) CreateUser(ctx context.Context, username, password string, rol
 	if len(username) < 2 || len(username) > 64 {
 		return User{}, errors.New("username must be 2-64 characters")
 	}
+	if !actors.IsPerson(username) {
+		return User{}, errors.New("username is reserved for internal components or contains ':'")
+	}
 	if !role.Valid() {
 		return User{}, fmt.Errorf("invalid role %q", role)
 	}