fix: reconnect reliably after radio restarts (TCP + serial)

[sg]

What changed

• Root cause: meshcore lib auto-reconnect tries 3×1s then gives up permanently; mcbot's DISCONNECTED handler only logged, leaving the bot alive but dead. Radio reboots take 5–60s, so recovery ~never happened.
• _on_disconnected (non-manual) now triggers the !adm restart teardown/rebuild path — every recovery is a full startup resync.
• amain(): signals (shutdown_event) now override internal restarts; unreachable radio retries with capped exponential backoff (5s→60s, forever) instead of exiting.
• New watchdog_interval registry setting (default 180s, 0=off): periodic device-query probe; 2 misses → restart. Catches half-open TCP sockets after silent power-cycles (no keepalive, idle bot may never send). Auto-appears in web UI/API/!adm setting.
• CONNECTED{reconnected} (lib recovered a short blip): refresh self_info + force contact re-sync.
• Docs: reconnection section in Bot_Usage, conf example entry.
• tests/test_reconnect.py (11 tests); suite now 96 passing.

Breaking changes

None. Serial unaffected except recovery now works; by-id device paths already recommended.

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