stop piling up idalib workers; raise max-workers headroom

Root cause of 'Maximum idalib worker count reached': the app bumped idle_ttl to
~never on every open, so sessions never freed and hit the default cap of 4.

- app: drop the immortal bump_idle_ttl(1e9); rely on the KeepAlive heartbeat
  (120s) to keep the running session warm, and open with a moderate idle_ttl
  (1800s) so the worker self-exits and frees its slot after the TUI closes.
- spawn.sh: set IDA_MCP_MAX_WORKERS (default 8) for headroom.
- doc: supervisor prunes unreachable workers before counting, so killing an idle
  worker frees a slot on the next open.

(Verified separately: big/truncated decompile results propagate renames fine via
the earlier force_recompile fix.)

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