fix(telegram): replace SIGTERM dedup with atomic lock + follower mode

[thewusman2025]

Summary

• Replaces the v0.0.5 bot.pid SIGTERM mechanism with an atomic exclusive-create lock file (O_CREAT|O_EXCL via fs.openSync('wx')). When a second instance starts and the lock holder is alive, it yields into follower mode instead of killing the existing poller — preserving the active MCP pipe.
• Follower mode: skips bot.start() (no polling), calls getMe() for bot identity, keeps all outbound MCP tools active (reply, edit_message, react, download_attachment) since they use bot.api.* which doesn't require a polling loop.
• Dead holder reclaim: if the lock file points to a dead PID (kill(pid, 0) throws), the stale lock is removed and the new instance becomes leader.
• Lock cleanup on shutdown: only removes poll.lock if owned by this process, matching v0.0.5's existing pattern.

Problem

The v0.0.5 bot.pid mechanism (#1349) SIGTERMs any existing poller on startup. This was intended to clear zombies, but it kills the MCP pipe of legitimate concurrent sessions — cron-fired sessions kill the interactive session's poller (#1360), and the dual-load bug (#1344) causes the second load to kill the first (#1363).

What changed

Before (v0.0.5)	After
Read bot.pid, SIGTERM holder, claim slot	Atomic openSync('wx') — fails if lock exists
Second instance always kills first	Second instance yields, runs follower mode
MCP pipe breaks on dual-load	MCP pipe preserved, outbound tools stay active
No follower concept	Leader polls, followers serve outbound tools

Test plan

• Single session: starts as leader, polls normally, lock file created
• Second session (same token): detects live leader, enters follower mode, outbound reply works
• Leader exits: lock file removed, next session becomes leader
• Orphan recovery: kill leader with SIGKILL (lock file stays), new session reclaims stale lock
• Dual-load (Telegram plugin: duplicate pollers cause background CPU churn even when idle #1344): second load yields without killing first

Fixes #1360, #1363. Related: #1344.

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