[arzafran]

Adds a setting to turn off the leftover agent process reaper. When an agent exits, Programa stops the processes it left behind (MCP servers and the like). Some people don't want that, so it can now be switched off.

• New app.reapLeftoverAgentProcesses setting, on by default, with a "Stop Leftover Agent Processes" toggle in Settings > App (English and Japanese).
• Off cancels the 10 second check and signals nothing. The interrupted-agent banner still works.
• Docs explain what gets stopped and what is kept.
• The reaper takes an injectable system seam, with 7 tests on a fake process table. The fake pids sit above the macOS limit, so nothing real can be signalled.
• Supervision finish loops now log per-record errors instead of dropping them.
• If the setting is turned off during the 3 second grace period, the final SIGKILL is skipped.

Not run locally; CI runs the tests.

Fixes INC-489