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/mcpd/tests/chat-service.test.ts

@@ -388,6 +388,8 @@ describe('ChatService', () => {
     expect(text.indexOf('HIGH prompt')).toBeLessThan(text.indexOf('LOW prompt'));
     // Agent's own system prompt leads.
     expect(text.indexOf('You are a helpful agent.')).toBeLessThan(text.indexOf('HIGH prompt'));
+    // The ambient context line (the model's clock) closes the block.
+    expect(text).toMatch(/\n\n\[context\] now: .+$/);
   });
 
   it('refuses tool calls when the agent has no project attached', async () => {