apps/browser/src/main/tab-manager.ts was 1288 lines, the largest src violation in the personal-agent fleet, exceeding the 450-line rule from AGENTS.md.

Split into apps/browser/src/main/tabs/ with tab-manager.ts as the thin orchestrator:

• apps/browser/src/main/tab-manager.ts: 389 lines (down from 1288)
• apps/browser/src/main/tabs/tab-navigation.ts: 277 lines
• apps/browser/src/main/tabs/tab-error-codes.ts: 197 lines
• apps/browser/src/main/tabs/tab-fullscreen.ts: 177 lines
• apps/browser/src/main/tabs/tab-page-loader.ts: 133 lines

All files now under 450 lines. IPC channel signatures preserved.

Verification:

• pnpm check-types: passed
• pnpm build: copy:assets step fails on Windows due to a pre-existing mkdir -p bash-ism, unrelated to this refactor

No IPC channel names, message shapes, or DRM-related code changed.