Task: Integrate agent browser tab focus fix

Identity

Integrate source a52e747 from task 20260927-browser-tab-focus-5c92d7a1 into local main. Promote its verified browser lifecycle/focus contract; no new product implementation.

Concurrent and Planning Gates Passed. Official start/status establish exclusive integration ownership at 220e899. Entry, own/source records, relevant positioning, decisions, architecture, domain and verification context loaded. Refreshed 123 peer records; packaging-170 is isolated and has no product edits. Historical placeholder records remain unknown without a concrete dependency.
User confirmed release --force of the completed teacher-read integration and --adopt-existing of the three known foreign documents. They remain unchanged and outside this commit.
Current user request supersedes the old all-tools-require-visible-view rule: only first viewport initialization requests foreground presentation. Keep explicit teardown, sandbox, generation/project and CDP guards.
Plan: fast-forward exact source, verify source/product equality, promote canonical documentation, pass drift/whitespace checks and commit. Reuse source validation if product bytes are identical. No push, deployment, build artifact, installation, cleanup or new reviewer.

Fast-forwarded local main from 220e899 to source a52e747a00 without conflicts. Product, tests, README and imported source record match the tested source exactly.
Promoted the first-viewport versus hidden-tab distinction and hidden-screenshot behavior into architecture/domain/current-state/history. Explicit browser close, project/module/window teardown and sandbox/CDP boundaries are unchanged.
The three adopted foreign task documents remain unchanged and uncommitted. No remote push, package, installation, production request or deletion.

Exact source ancestry and product/test/README/source-record equality passed; no merge resolution or product edit occurred.
Reused source evidence: 134 related unit tests, 4 Electron scenarios, standard typecheck and Vite build passed. Extra Main-wide compiler check retains 66 baseline diagnostics, zero additions and none in touched files.
Whitespace and official task-aware documentation checks passed; adopted documents retain their recorded original states.

Source worktree remains available pending the separate cleanup confirmation. Installed client still needs a rebuilt package to receive the fix.

Source promotion accepted under the user's browser-fix and merge authorization; applied to module-map, data-flow, system-overview and business-rules. Older visibility wording is clarified; no unresolved product decision or source-record edit.