fix: release host terminals after usage scans

[MayankBansal12]

Summary

• let host scan commands exit naturally instead of holding their shells with an infinite sleep loop
• read final output and exit status after the terminal exits
• use clean close for completed commands, reserving force close for timeout or cancellation
• log terminal cleanup failures
• cap collection at three concurrent terminals per machine

Why

On bb 0.40.0 for macOS, repeated forced closes can leak PTY resources. The prior keep-alive loop and nine parallel scans accelerated that leak until bb could no longer open terminals.

Fixes #28

[MayankBansal12]

Follow-up validation: bb only exposes terminal output while a session is running, so reading after natural exit caused HTTP 409 failures across all collectors. The branch now uses a bounded handshake: capture output while running, send a release line, then wait for natural exit. The shell fallback is capped at 30 seconds—there is no infinite hold.

Live-tested with an isolated copy of the actual plugin restricted to the current machine. Codex, Claude, fx, Grok, Pi, Prime, Antigravity, OpenCode, and OpenCode Go all completed without terminal-output 409s. The temporary test plugin was removed afterward.