docs: recommend Git Bash as default Windows shell for flex-ax

[longfin]

Summary

• Remove install.ps1 and install-common.ps1 — PowerShell install scripts 삭제
• README에 Windows 섹션 추가: Git Bash 기본 셸 권장 + gws JSON quoting 트러블슈팅
• playwriter-setup.md Windows 노트에서 PowerShell 언급 제거
• install-common.sh 주석에서 PowerShell 언급 제거

Refs #39

Test plan

• release-pack.sh --camp flex-ax 빌드 성공
• 릴리스 에셋에 .ps1 파일 미포함 확인
• install-common.sh, camp tarball 내 PowerShell 언급 0건 확인
• 테스트 릴리스: flex-ax-v1.3.0-rc.1

🤖 Generated with Claude Code

[longfin]

RC test feedback from Windows (flex-ax-v1.3.0-rc.1): this PR is moving in the right direction.

What I verified locally:

• install completed successfully when CAMP_VERSION=v1.3.0-rc.1 was set explicitly
• generated wrappers worked: flex-ax.cmd, gws.cmd, gws-auth.cmd, playwriter.cmd
• generated camp files existed: AGENTS.md, manifest.yaml, scripts/check-freshness.sh
• flex-ax --version worked on Windows (flex-ax 0.2.1)

Also confirmed: the current generated AGENTS.md already includes the right high-level guidance:

• Use Git Bash (or MSYS2 / WSL) as the default shell on Windows.
• All CLI commands and examples in this camp assume a bash-compatible shell.

That matches what we saw in practice.

One remaining docs nit that still feels important: many generated gws-* skill examples and startup examples still look like this:

gws gmail users getProfile --params '{"userId":"me"}'

Those snippets are valid in Git Bash, but they are still easy to copy into PowerShell by mistake. In testing, PowerShell continued to fail with:

Invalid --params JSON: key must be a string at line 1 column 2

So I think the remaining opportunity here is not the overall direction, but making the shell assumption even harder to miss near the first gws examples. Something like:

• Examples using --params / --json below are bash examples.
• On Windows, do not run JSON-heavy gws commands from PowerShell.
• If the current shell is PowerShell, switch to Git Bash or use bash -lc.

One separate thing I noticed during RC install testing: the tagged camps/flex-ax/install.sh still had CAMP_VERSION defaulting to v1.2.0, so I had to set CAMP_VERSION=v1.3.0-rc.1 explicitly to test the RC assets. If that is unintended, it may be worth fixing before the final release.