Replace clone-swift.sh with git submodule for Swift headers

[Kyle-Ye]

Summary

• Add Checkouts/swift as a git submodule pointing to OpenSwiftUIProject/swift-runtime-headers (release/6.1 branch)
• Add submodules: true to all actions/checkout@v4 steps in CI workflows
• Remove Scripts/clone-swift.sh, Plugins/CloneSwiftPlugin, and .github/actions/checkout-swift-headers — no longer needed
• Bump version to 6.5.1 and enable apple-generic versioning

Test plan

• Verify CI workflows pass with submodule checkout
• Verify git clone --recurse-submodules works for fresh clones
• Verify swift build succeeds without --disable-sandbox

[augmentcode]

🤖 Augment PR Summary

Summary: This PR replaces the Swift-header bootstrap mechanism with a pinned git submodule.

Changes:

• Add Checkouts/swift as a submodule pointing to OpenSwiftUIProject/swift-runtime-headers (release/6.1) and pin the commit.
• Update CI workflows to checkout submodules via actions/checkout@v4 (submodules: true).
• Remove the custom checkout action (.github/actions/checkout-swift-headers), Scripts/clone-swift.sh, and the SwiftPM build-tool plugin (CloneSwiftPlugin).
• Adjust .gitignore so only the Swift-headers submodule is kept under Checkouts/.
• Update README instructions to initialize the headers via git submodule update --init.

Technical Notes: Builds now assume headers are already present at Checkouts/swift (no plugin-driven cloning / sandbox workaround).

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[augmentcode]

Review completed. 1 suggestions posted.

Comment augment review to trigger a new review at any time.

[augmentcode]

Consider documenting git submodule update --init --recursive (and/or git clone --recurse-submodules) here so nested submodules won’t be missed if they’re introduced later.

Severity: low

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 31.47%. Comparing base (6c04001) to head (06cd691).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #217      +/-   ##
==========================================
- Coverage   32.08%   31.47%   -0.62%     
==========================================
  Files          70       70              
  Lines        2596     2596              
==========================================
- Hits          833      817      -16     
- Misses       1763     1779      +16     

Flag	Coverage Δ
ios	31.48% <ø> (-0.62%)	⬇️
macos	9.95% <ø> (ø)
ubuntu	0.00% <ø> (ø)

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see 2 files with indirect coverage changes

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