ambient context: give every project's model a clock

Models consuming projects have no idea what day it is. Three always-on
layers from mcplocal plus one from mcpd:

- initialize instructions and the begin_session briefing carry
  '[context] now: <date/time> — project: <name>' (frozen at session
  start by design);
- a get_current_time virtual tool (ungated catalogs only, same pattern
  as read_prompts);
- every tools/call result gets a trailing '[context] now: …' content
  item at the endpoint send site — the only chokepoint that also covers
  virtual tools, gate intercepts and failure results, and the layer
  that keeps a days-old chat session current;
- mcpd agent/project chat system blocks append the same line each turn.

Time renders as UTC plus an optional local timezone from
MCPLOCAL_AMBIENT_TZ (IANA name; both services read it; invalid or unset
means UTC only). ~15 tokens per stamp, minute precision.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/mcplocal/tests/plugin-gate.test.ts

@@ -199,6 +199,19 @@ describe('Gate Plugin via setPlugin()', () => {
   });
 
   describe('begin_session', () => {
+    it('opens the briefing with the ambient context line (the model has no clock)', async () => {
+      const { router } = setupPluginRouter();
+      await router.route({ jsonrpc: '2.0', id: 1, method: 'initialize' }, { sessionId: 's1' });
+
+      const res = await router.route(
+        { jsonrpc: '2.0', id: 2, method: 'tools/call', params: { name: 'begin_session', arguments: { tags: ['zigbee'] } } },
+        { sessionId: 's1' },
+      );
+
+      const text = ((res.result as { content: Array<{ text: string }> }).content[0]!.text);
+      expect(text).toMatch(/^\[context\] now: /);
+    });
+
     it('falls back with a visible warning when the LLM selector errors (no silent degrade)', async () => {
       const { router } = setupPluginRouter({ withLlm: true, llmError: true });
       await router.route({ jsonrpc: '2.0', id: 1, method: 'initialize' }, { sessionId: 's1' });