fix(agent): purge sync.Once from pkg/agentic via core.Once (§14A)

Closes Mantis #863 ([agent] Phase 2: purge sync stdlib).

Per RFC plans/code/core/go/RFC.primitives-lifecycle.md §14A (landed core/go
dev 8995a80), swaps the four sync.Once usages to core.Once and the two
sync.Once{} reset-pattern callsites to core.Once.Reset():

pkg/agentic/statestore.go:
- Drop `import "sync"`.
- stateStoreRef.once: sync.Once → core.Once
- closeStateStore reset: `s.stateOnce = sync.Once{}` → `s.stateOnce.Reset()`

pkg/agentic/workspace_stats.go:
- Drop `import "sync"`.
- workspaceStatsRef.once: sync.Once → core.Once
- closeWorkspaceStatsStore reset: `s.workspaceStatsOnce = sync.Once{}` →
  `s.workspaceStatsOnce.Reset()`

pkg/agentic/prep.go:
- Drop `import "sync"`.
- PrepSubsystem.stateOnce + .workspaceStatsOnce: sync.Once → core.Once

The Reset() pattern matches stdlib semantics (see RFC §14A "Tradeoff: Once.
Reset semantics") — caller serialises via the existing closeStateStore /
closeWorkspaceStatsStore structure that nests Reset inside the lifecycle
inverse, so no concurrent Do races are introduced.

Net: 3 files, +7/-11. Mechanical line-edit per RFC §16 migration plan.

Audit re-check post-commit:
  grep -n '"sync"\|sync\.Once\|sync\.Mutex' pkg/agentic/{statestore,workspace_stats,prep}.go
  → empty (lib local variable named `sync` in mirror.go is unrelated; not
    in scope of this ticket).

Pre-flight verification: core.Once + Reset symbols verified present on
core/go dev 8995a80. Local AX-10 build blocked by the same pre-existing
workspace forge dep break that affects all consumers (root cause: fake
v0.8.0-alpha.1 pins per task #28); CI in healthy env will validate.

Co-Authored-By: Athena <athena@lthn.ai>
Co-Authored-By: Virgil <virgil@lethean.io>

pkg/agentic/prep.go

@@ -8,7 +8,6 @@ import (
 	"crypto/sha256"
 	"encoding/base64"
 	"encoding/hex"
-	"sync"
 	"time"
 
 	"dappco.re/go/agent/pkg/lib"
@@ -41,9 +40,9 @@ type PrepSubsystem struct {
 	failCount          map[string]int
 	providers          *ProviderManager
 	workspaces         *core.Registry[*WorkspaceStatus]
-	stateOnce          sync.Once
+	stateOnce          core.Once
 	state              *stateStoreRef
-	workspaceStatsOnce sync.Once
+	workspaceStatsOnce core.Once
 	workspaceStats     *workspaceStatsRef
 }
 

pkg/agentic/statestore.go

@@ -3,8 +3,6 @@
 package agentic
 
 import (
-	"sync"
-
 	core "dappco.re/go/core"
 	store "dappco.re/go/store"
 )
@@ -28,9 +26,9 @@ func stateStorePath() string {
 }
 
 // stateStoreRef keeps the store instance, its initialisation error, and a
-// sync.Once so multiple callers observe the same lazily-initialised value.
+// core.Once so multiple callers observe the same lazily-initialised value.
 type stateStoreRef struct {
-	once     sync.Once
+	once     core.Once
 	instance *store.Store
 	err      error
 }
@@ -102,7 +100,7 @@ func (s *PrepSubsystem) closeStateStore() {
 	}
 	ref.err = nil
 	s.state = nil
-	s.stateOnce = sync.Once{}
+	s.stateOnce.Reset()
 }
 
 // openStateStore attempts to open the canonical state store at

pkg/agentic/workspace_stats.go

@@ -3,7 +3,6 @@
 package agentic
 
 import (
-	"sync"
 	"time"
 
 	core "dappco.re/go/core"
@@ -25,7 +24,7 @@ const stateWorkspaceStatsGroup = "stats"
 // separate from the top-level `stateStoreRef` so the two stores open
 // independently — a missing parent DB does not disable top-level state.
 type workspaceStatsRef struct {
-	once     sync.Once
+	once     core.Once
 	instance *store.Store
 	err      error
 }
@@ -90,7 +89,7 @@ func (s *PrepSubsystem) closeWorkspaceStatsStore() {
 	}
 	ref.err = nil
 	s.workspaceStats = nil
-	s.workspaceStatsOnce = sync.Once{}
+	s.workspaceStatsOnce.Reset()
 }
 
 // openWorkspaceStatsStore opens the parent workspace stats database,