fix(telegram): keep typing indicator alive while Claude is processing

[snemesh]

Problem

Telegram's typing indicator expires after ~5 seconds. For queries that take 30s–2min (tool use, file reads, complex reasoning), the indicator disappears and the user can't tell if the bot is still working or has silently failed. This is especially problematic because --channels is experimental and sessions can genuinely stop responding (#777, #963).

Fix

Added startTyping(chat_id) / stopTyping(chat_id) helpers:

• startTyping sends sendChatAction("typing") immediately, then repeats every 4 seconds via setInterval
• stopTyping clears the interval — called when reply or edit_message sends a response
• Multiple chats tracked independently via Map<string, interval>
• startTyping on the same chat replaces the previous interval (no leak)
• stopTyping on unknown chat is a safe no-op

Changes

• external_plugins/telegram/server.ts — 27 insertions, 2 deletions

Test plan

• bun build passes
• 12/12 unit tests pass

Test output (12/12 passed)

── Test 1: immediate typing on startTyping ──
  ✅ Typing sent immediately
  ✅ Correct chat_id
  ✅ Interval registered

── Test 2: typing repeats every interval ──
  ✅ Typing sent multiple times (got 3, expect 3-4)

── Test 3: stopTyping stops the indicator ──
  ✅ No more typing after stop (before=2, after=2)
  ✅ Interval removed from map

── Test 4: multiple chats are independent ──
  ✅ Two intervals active
  ✅ Only chat_B remains
  ✅ All cleared

── Test 5: startTyping on same chat replaces interval ──
  ✅ Interval replaced (not duplicated)
  ✅ Different interval object

── Test 6: stopTyping on unknown chat is no-op ──
  ✅ No error thrown

═══ Result: 12/12 passed ✅ ═══

Fixes #932