feat: native Windows launchers (PowerShell/cmd) + install.ps1

[tizz98]

Summary

Makes ./meta <cmd> work transparently on native Windows, not just Git Bash.

• meta.cmd + .meta/shim.ps1 (new generated artifacts): PowerShell (./meta) and cmd.exe (.\meta) both resolve meta.cmd, which relaunches PowerShell with -ExecutionPolicy Bypass into the hidden .meta/shim.ps1 (mirror of the bash shim.sh: read .meta/version → map arch → download the pinned release → verify sha256 → exec → propagate exit code). The bash meta shim is unchanged.
• install.ps1 (new): native irm https://raw.githubusercontent.com/tizz98/ai-meta/main/install.ps1 | iex bootstrapper, mirror of install.sh.
• .gitattributes: pins *.cmd→CRLF and the bash-path files (*.sh, meta, .meta/version)→LF, so a Windows checkout never bakes a CRLF-broken bash shim into the binary via include_str!.
• Dogfood: this repo carries the launchers and is pinned to v0.4.0 (the first release with a Windows asset) so ./meta resolves a binary in-repo.

Design (why no root meta.ps1)

PowerShell ranks a repo-root .ps1 above a PATHEXT .cmd, and a directly-invoked .ps1 is subject to execution policy. Keeping no root meta.ps1 makes PowerShell's ./meta fall through to meta.cmd, whose -ExecutionPolicy Bypass is immune to the machine policy. Verified empirically on PowerShell 5.1 and 7.

Test plan

• cargo test — new scaffold::tests::generates_windows_launchers passes. (102 pass; the 2 failing src/process.rs tests are a pre-existing sh-on-Windows env issue, green on CI/Linux, untouched by this branch — follow-up chipped.)
• cargo clippy --all-targets -- -D warnings, cargo fmt --all — clean.
• E2E on Windows in a scratch repo: ./meta check under Restricted execution policy → exit 0 (downloaded the real ai-meta-x86_64-pc-windows-msvc.exe); .\meta in cmd.exe works; a nonzero exit propagates through both hops exactly once (no double-run).
• install.ps1 real install → runnable meta.exe; checksum verified; PATH nudge is print-only.
• CI green on ubuntu.

Follow-ups (not in this PR)

• Generate a .gitattributes into managed repos (so a consumer's committed bash shim survives a Windows teammate's clone).
• Fix which() PATHEXT resolution + the two sh-dependent src/process.rs tests on Windows.

🤖 Generated with Claude Code