Memory Leak Reporter Improvements

Reduced noise from auto-reported memory leak issues by ~90%.

Changes

• Raised reporter threshold from 200MB → 400MB — the self-healing restart mechanism already handles growth at 200MB when idle, so we only report when self-healing failed
• Added session gate — skip reports when sessions=0 and browser is already dead (restart handled it). Extreme growth (>600MB) still reports as a safety net.
• Capture browser RSS during growth — lastSeenBrowserRssMb is now captured while the browser is alive (previously always null by report time). Enables distinguishing Node/Playwright leaks from Firefox shared-memory bleed.
• Auto-close bot now handles memory-leak issues with 0 contexts + 0 tabs (same pattern as stuck-issue handling)
• Fixed misleading code comment — native memory metric measures Node/Playwright state (CDP buffers, glibc arenas), not Firefox's jemalloc

Context

Audit of 78 auto-reported memory-leak issues found all were self-healing conditions generating noise. The memory pressure restart (kills browser at 200MB growth when idle) was already handling these, but the reporter fired at the same threshold, creating duplicate signals for resolved conditions.