chore: clarify E2E test comments

[Kevinjohn]

Summary

• Clarify E2E comments around browser fixture and test intent.
• Let unexpected Playwright errors surface instead of converting them into false results.

Validation

• pnpm run gate:all passed.
• pnpm run e2e passed (294 tests).

Merge predecessor: none

Closes #1469

Merge order

#1472 → #1473, #1474, #1466

• Set body text to 13px #1465 and chore: align repository configuration comments #1471 have merged. No textual conflicts with main (2950329) or between any pair of the remaining PRs. All four merged in this order onto main cleanly, and the combined tree passes format, typecheck, lint and the docs build (tree clean).
• Rewrite the documentation front door around four role quick starts #1466 lands last because it carries regenerated docs/ output.
• Each PR will be behind main once its predecessor lands; update the branch from main before merging.

Merge predecessor for this PR: none.

Conflicts: none.

[Kevinjohn]

Polish review: no P1 or P2 findings, no changes pushed.

• Comment-stripped AST comparison across the 54 changed files finds one behaviour change: waitForAppLanding in e2e/browserTestSupport.ts drops .catch(() => false) from isVisible(). Playwright's isVisible() returns false for a missing element, so it now throws only on a strict-mode or closed-page error, which should fail the test.
• Ladder passed on Node 24 at the PR head: pnpm run check:push (format, typecheck, lint with zero warnings, file-size policy) and pnpm run policy:comment-voice. (typecheck covers the e2e project.)
• Integration: chore: align repository configuration comments #1471, chore: align server, shared, and script comments #1472, fix: surface unexpected repeat time-off failures #1473, chore: clarify E2E test comments #1474 and Rewrite the documentation front door around four role quick starts #1466 merged in that order onto main at 8141c39 without conflicts; check:push, policy:comment-voice, docs:build (tree clean afterwards) and the documentation, runbook and user-story suites (430 tests) pass on the combined tree.

Merge predecessor: none.