docs: fix inaccuracies found during documentation accuracy review

- CHANGELOG: inactivity_kill_secs default is disabled (None), not 120
- TROUBLESHOOTING: clarify inactivity_kill_secs is opt-in (must be
  passed explicitly); correct "default 120" claim
- MCP_TOOLS: inactivity_kill_secs is disabled by default; add note
  that Wine/macOS may not exit naturally even with ShutdownTerminal=1
- handlers/backtest.rs: remove stale comment claiming inactivity
  watchdog is "intentionally skipped when shutdown=true" — replaced
  with accurate description of the new 30s HTML-wait + kill behavior

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -17,8 +17,8 @@
   `find_active_tester_agent_log` now prefers `Agent-127.0.0.1` and tiebreaks by file size.
 
 ### Added
-- `launch_backtest` exposes `shutdown` (default `true`) and `inactivity_kill_secs` (default `120`)
-  as explicit tool parameters.
+- `launch_backtest` exposes `shutdown` (default `true`) and `inactivity_kill_secs` (default: disabled;
+  set to e.g. `120` to enable the inactivity watchdog) as explicit tool parameters.
 - Report DB stores deals in SQLite; all analytics tools resolve by `report_id` / `report_dir` / latest.
 
 ### Verified