rewriter: auto-rewrite xcodebuild → rtk xcodebuild (filter exists since v0.30.0)

Context

Issue #376 was closed COMPLETED in v0.30.0 with a xcodebuild.toml TOML filter — rtk xcodebuild build correctly strips CompileSwift lines etc.

However, the rewriter registry (rtk rewrite) does not auto-rewrite raw xcodebuild invocations to rtk xcodebuild. This means when xcodebuild is called from a Claude Code Bash hook (or any wrapper that uses rtk rewrite), the command falls through unchanged and the filter never fires.

Reproduction (rtk 0.34.2)

$ rtk rewrite "xcodebuild build -project Foo.xcodeproj"; echo $?
1                                       # exit 1 = "no RTK equivalent" → passthrough

$ rtk rewrite "git status"; echo $?
rtk git status
0                                       # exit 0 = rewrite applied

$ rtk xcodebuild build -project Foo.xcodeproj   # explicit prefix works fine
... (filtered output)

Impact

From a single Claude Code user's rtk discover over 30 days:

xcodebuild   3,231 invocations    (top unhandled command after ssh)

That's ~3,231 calls/month per user that bypass the v0.30.0 filter purely because the rewriter doesn't add the prefix.

Suggested fix

Add xcodebuild (and probably xcrun, swift-already-supported, etc.) to the rewriter registry in src/discover/registry.rs so it returns exit 0 with "rtk xcodebuild $args". The filter is already there — this is just wiring the rewriter side.

Verification of fix

$ rtk rewrite "xcodebuild build -project Foo.xcodeproj"
rtk xcodebuild build -project Foo.xcodeproj
$ echo $?
0