fix(setup): Playwright Chromium failure becomes best-effort warn, not exit 1

[DavidMiserak]

Carving out the Playwright-warn change from #1883 as a focused PR per
@jbetala7's review request. Coordinated with #1838 — see "Relationship
to #1838" below.

Problem

The Playwright Chromium install in ./setup is the riskiest step in the
whole script: it needs network, talks to a Microsoft CDN, on Windows it
bounces through Node.js because of a Bun pipe bug, and on locked-down
corporate machines it may not even be allowed to run. Until now, any
failure in that step was fatal (exit 1) and aborted ./setup mid-run,
before skills got registered, hooks got installed, or migrations ran.

A re-run would start over from the top, hit the same Chromium download
issue, fail the same way. There was no clean way to get a working gstack
install on a machine where Chromium was unreachable, even though every
non-browser feature would have worked fine.

Fix

Replace both exit 1 paths in the Playwright install/verify block with a
$_PW_FAIL_REASON accumulator. On failure, the script:

• Prints a named warning to stderr identifying which sub-step failed:
  chromium-install, windows-no-node, windows-node-modules, or
  post-install-launch.
• Lists the affected browser-driven features (/qa, /design-review,
  make-pdf, sidebar, /pair-agent).
• On Windows, includes the Bun-on-Windows pipe-bug link and the
  node -e "require('playwright')" verify command.
• Tells the user to re-run ./setup to retry — at which point only the
  Playwright step needs to succeed, because skills, hooks, and
  migrations are already in place from earlier steps.

All other setup behavior is unchanged. The script exits 0 in the
warn-only path and the user gets a usable gstack install minus browser
features.

Tests

test/setup-playwright-warn-not-exit.test.ts — 5 tests, exit-code-path
exercised (not source-grep):

1. No exit 1 reachable from the block — source-anchored regex
   on the live block extracted from setup.
2. _PW_FAIL_REASON accumulator + named warning present — same
   source-anchored extraction.
3. Functional: install failure → exit 0 + warning. Runs the live
   block in bash with stubbed ensure_playwright_browser (always fails)
   and stubbed bunx (fails). Asserts script exits 0, stderr contains
   warning: Playwright Chromium is unavailable (chromium-install),
   and the retry hint fires.
4. Side-by-side: old bug shape exits 1. Inlines the OLD exit 1
   path with the same stubs and confirms it returned non-zero. Proves
   the exit-code difference is real, not a quoting illusion.
5. Functional: post-install launch failure also degrades to warn.
   bunx succeeds, ensure_playwright_browser keeps failing — asserts
   exit 0 with reason post-install-launch.

$ bun test test/setup-playwright-warn-not-exit.test.ts
 5 pass / 0 fail / 14 expect() calls

$ bun test test/setup-*.test.ts
 70 pass / 0 fail / 165 expect() calls

Test plan

• bun test test/setup-playwright-warn-not-exit.test.ts — 5 pass
• bun test test/setup-*.test.ts — 70 pass
• bash -n setup — syntax OK
• Manually traced the new path for each _PW_FAIL_REASON value

Relationship to #1838

This PR and #1838 modify the same Playwright block in setup but
solve non-overlapping problems:

PR	What it changes
#1838	Which Chromium version gets installed (pins bunx playwright install to the bundled playwright/package.json version so the install-time and runtime versions don't drift) + tightens ensure_playwright_browser() to assert executablePath() exists.
this PR	What happens when install fails (exit 1 → _PW_FAIL_REASON accumulator + named warning, no abort).

Both can land — they're complementary. Whichever lands first, the other
needs a small rebase. I'm happy to rebase this PR onto #1838 if the
maintainers prefer that order (likely the cleaner sequence, since #1838
is closer to merging).

Relationship to #1883

This PR is the "(2) Playwright warn-don't-exit" split @jbetala7 asked
for on #1883. The third split — the phase reorder — will be a separate
PR against fresh main with an issue describing the failure modes it
fixes. #1883 stays open until all three splits are visible, then I'll
close it with links to all three.

Sibling: #1898 (the two bug fixes, opened first).

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