fix: use consecutive-old-counter for telegram dialog cutoff

[dandaka]

Summary

Fixes #15 — Telegram sync was missing chats with new messages because iter_dialogs() ordering is not strictly by last_message_date.

• Replace immediate break on first old dialog with a consecutive-old-counter (threshold=100)
• Old dialogs are skipped but scanning continues through the blurred zone
• Only stops after 100 consecutive old dialogs (one full Telethon internal page)

Root Cause

Telethon's iter_dialogs() returns dialogs sorted by Telegram's internal "last activity" order, which includes pinned status, admin actions, etc. — not strictly by last_message_date. This creates a blurred boundary zone where chats with and without new messages are intermixed.

The previous code did break on the first dialog older than the --since cutoff. If a single old dialog appeared before newer ones (e.g., a pinned chat), all subsequent dialogs were hidden — including chats with messages from hours ago.

Production evidence (2026-03-18):

• Cutoff: 13:05:54Z
• Dialog fix: WhatsApp auth - command injection, wrong endpoint, 422 handling #1: "Настя Же" → 13:04:23 (1 min before cutoff) → broke immediately
• Dialog ci: add GitHub Actions workflow for tests and type checking #2: "Слава Мишаков" → 18:01:40 (never seen)
• Dialog docs: add WhatsApp/WAHA setup guide #3: "Portugal..." → 18:55:10 (never seen)
• Result: 0 chats returned, entire sync cycle was a no-op

Performance

Approach	Dialogs scanned	Time
Previous (broken)	1	<1s
Full scan (no cutoff)	3,334	~35s
This fix	~300-500 typical	~3-5s

Test plan

• Restart daemon, verify chats that were previously missed now sync
• Check traul messages -c "AI Agents Lab 01" -l 5 shows today's messages after next sync cycle
• Monitor daemon logs for "consecutive old" stop message to confirm early exit works
• Verify no FloodWaitError from Telegram after several sync cycles

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