feat: timezone-aware context injection for agent prompts (#691)

* feat: per-group timezone architecture with context injection (#483)

Implement a comprehensive timezone consistency layer so the AI agent always
receives timestamps in the user's local timezone. The framework handles all
UTC↔local conversion transparently — the agent never performs manual timezone
math.

Key changes:
- Per-group timezone stored in containerConfig (no DB migration needed)
- Context injection: <context timezone="..." current_time="..." /> header
  prepended to every agent prompt with local time and IANA timezone
- Message timestamps converted from UTC to local display in formatMessages()
- schedule_task translation layer: agent writes local times, framework
  converts to UTC using per-group timezone for cron, once, and interval types
- Container TZ env var now uses per-group timezone instead of global constant
- New set_timezone MCP tool for users to update their timezone dynamically
- NANOCLAW_TIMEZONE passed to MCP server environment for tool confirmations

Architecture: Store UTC everywhere, convert at boundaries (display to agent,
parse from agent). Groups without timezone configured fall back to the server
TIMEZONE constant for full backward compatibility.

Closes #483
Closes #526

Co-authored-by: shawnYJ <shawny011717@users.noreply.github.com>
Co-authored-by: Adrian <Lafunamor@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

* style: apply prettier formatting

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

* refactor: strip to minimalist context injection — global TIMEZONE only

Remove per-group timezone support, set_timezone MCP tool, and all
related IPC handlers. The implementation now uses the global system
TIMEZONE for all groups, keeping the diff focused on the message
formatting layer: mandatory timezone param in formatMessages(),
<context> header injection, and formatLocalTime/formatCurrentTime
helpers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* refactor: drop formatCurrentTime and simplify context header

Address PR review: remove redundant formatCurrentTime() since message
timestamps already carry localized times. Simplify <context> header to
only include timezone name.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: shawnYJ <shawny011717@users.noreply.github.com>
Co-authored-by: Adrian <Lafunamor@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/timezone.test.ts

@@ -0,0 +1,29 @@
+import { describe, it, expect } from 'vitest';
+
+import { formatLocalTime } from './timezone.js';
+
+// --- formatLocalTime ---
+
+describe('formatLocalTime', () => {
+  it('converts UTC to local time display', () => {
+    // 2026-02-04T18:30:00Z in America/New_York (EST, UTC-5) = 1:30 PM
+    const result = formatLocalTime(
+      '2026-02-04T18:30:00.000Z',
+      'America/New_York',
+    );
+    expect(result).toContain('1:30');
+    expect(result).toContain('PM');
+    expect(result).toContain('Feb');
+    expect(result).toContain('2026');
+  });
+
+  it('handles different timezones', () => {
+    // Same UTC time should produce different local times
+    const utc = '2026-06-15T12:00:00.000Z';
+    const ny = formatLocalTime(utc, 'America/New_York');
+    const tokyo = formatLocalTime(utc, 'Asia/Tokyo');
+    // NY is UTC-4 in summer (EDT), Tokyo is UTC+9
+    expect(ny).toContain('8:00');
+    expect(tokyo).toContain('9:00');
+  });
+});