fix: resilient two-phase setup — skills install without bun/Playwright

[jteng]

Summary

• Restructures ./setup into Phase 1 (skill linking — zero deps beyond bash+git) and Phase 2 (browse build + Playwright — requires bun)
• Phase 1 always runs first, so Claude skills are linked on any system — even corporate laptops that can't install bun or Playwright
• Phase 2 failures produce warnings, not fatal exits. Unified summary at end reports what succeeded

What changed

• Fatal bun check → conditional HAS_BUN flag
• Skill linking moved before browse build
• Codex/Factory linking guarded on .agents//.factory/ directory existence
• Kiro browse symlinks guarded on browse/dist existence
• Phase 2 wrapped in set +e / set -e for error tolerance
• 8 new structural tests for phase ordering and resilience

Test plan

• bun test passes (633 tests, 0 failures)
• Run ./setup without bun → links Claude skills, warns about rest, no fatal exit
• Run ./setup with bun but Playwright fails → no exit 1 in browse build or Playwright sections
• Run ./setup normally → identical behavior to current, plus unified summary

🤖 Generated with Claude Code

[time-attack]

@16francej — recommend closing this PR as superseded by #2233. It is far behind current setup behavior and has failure-handling defects; #2233 is the current containment work, while broader no-Bun support requires a separate product decision. Credit #683 for the earlier skill-survival insight, but do not carry over the branch code.

[capy-ai]

Thanks for this, @jteng. The fix is now on main: setup was restructured into phases with staged swaps, so this rewrite no longer applies (see v1.91.16.0, #3014). Closing as superseded; if you still see the problem on the latest gstack, please open an issue with ./setup --status output.