feat(kb): wire kb-block fade into before_prompt_build

The previous commit shipped renderFaded + tick + seed on Block but
left them dead code because openclaw's `before_prompt_build` hook
ctx doesn't carry a currentTurn signal — the hook called plain
render() and turnFor() returned 0, so add() always stamped
last_refresh_at_turn=0 and fade distance never moved.

This adds a small per-session turn tracker (tools/turn-tracker.ts)
that the hook bumps each fire. On the first bump after plugin start
the counter is seeded from max(entry.last_refresh_at_turn) on the
loaded block, so a restart doesn't regress accumulated fade for
surviving entries. dynamic-kb-cache now reads currentTurnForSession
via the same counter so a fact cached on turn N reads back at d=0
on turn N+1.

The hook now: bumps turn, ticks (drop entries past the m% threshold,
log + save when anything drops), and renderFaded() into
appendSystemContext. Same algorithm as the Plexum-side
RenderDynamicSubblock path.

KBDeps.turnFor signature changed from (agentId) → (sessionId) since
turn is session-scoped — fixed the one cache-tool call site.

Verified end-to-end with a standalone smoke replaying the full
hook+tool flow against a temp profile: stale entry (d=201) drops on
turn 1, fresh entry (d=1) survives untouched, restart re-seeds the
counter from the surviving entry's last_refresh.

plugin/tools/dynamic-kb.ts

@@ -39,8 +39,13 @@ export interface KBDeps {
   tokenFor: ((agentId: string) => Promise<string | null>) | null;
   /** Build new client given a token (so each call gets fresh per-agent client). */
   makeClient: ((token: string) => KBClient) | null;
-  /** Best-effort current turn number; 0 is acceptable. */
-  turnFor: (agentId: string) => number;
+  /**
+   * Best-effort current turn number for a session; 0 is acceptable.
+   * Wired against the same per-session counter that the
+   * before_prompt_build hook bumps, so a fact added during turn N
+   * reads back at d=0 on turn N+1.
+   */
+  turnFor: (sessionId: string) => number;
 }
 
 /**
@@ -211,7 +216,7 @@ export function createCacheTool(deps: KBDeps) {
           return err('dynamic-kb-cache: ' + (e?.message || String(e)));
         }
       }
-      const turn = deps.turnFor(ctx.agentId);
+      const turn = deps.turnFor(ctx.sessionId);
       const fetchedByID = new Map(fetched.map((f) => [f.id, f]));
       const added: number[] = [];
       for (const id of toFetch) {