ci: dedupe CmuxTerminalCore regression execution

[teamleaderleo]

Summary

Remove the app-host shard's standalone CmuxTerminalCore-Package build-for-testing + xcrun xctest gate. The required swift-package-tests job already runs swift test for CmuxTerminalCore, which executes the same package test target with strict failure handling.

This keeps one owner for the package tests and removes an independently linked copy of the same test product.

Measurement

On green full-suite run 35427062807, Run Ghostty split-theme appearance regression took 20s on shard 6. A recent post-balance canary showed the same step at about 20s.

• focused package builds: 2 → 1 owner (swift-package-tests)
• app-host test-host/build launches: -1
• shard 6 reserved focused wall time: 284s → 264s
• package semantic coverage: unchanged; CmuxTerminalCore remains in the strict package list

The package selector already treats .github/workflows/ci.yml as a global package-test input, so this PR's hosted validation will execute the package gate.

Failure accounting

No retry or tolerance changes. A real CmuxTerminalCore compile/test failure still fails swift-package-tests and therefore the required tests gate.

Stacked on #13386 so the diffs stay focused.

Related: #6134, #13095, #13325.

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Summary by cubic

Removes the app-host shard's duplicate CmuxTerminalCore test run so each package test executes once.

• swift-package-tests already runs CmuxTerminalCore, so the shard's xcodebuild + xctest step was a redundant, separately linked copy.
• Shard 6's reserved focused wall time drops from 284s to 264s.
• Adds test assertions to keep CmuxTerminalCore coverage owned by the strict package gate.

^{Written for commit 2d792d2. Summary will update on new commits.}

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[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge because the removed regression execution remains covered by the required Swift package test job.

Summary

This PR removes the duplicate app-host execution of the CmuxTerminalCore regression suite and keeps its coverage in the strict Swift package test job.

• Removes the standalone xcodebuild and xctest invocation from app-host shard 6.
• Reduces shard 6's reserved wall-time estimate from 284 to 264 seconds.
• Adds a targeted workflow guard that verifies CmuxTerminalCore remains an exact member of the package test array.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart LR
    CI[CI workflow] --> PackageGate[swift-package-tests]
    PackageGate --> Selector[Package selector]
    Selector --> Core[swift test CmuxTerminalCore]
    CI --> AppHost[app-host-unit-tests]
    AppHost --> AppTests[Restored app-host test product]

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_{Reviews (2) · Last reviewed commit: "test(ci): pin CmuxTerminalCore package o..."}

[teamleaderleo]

Superseded by #13427, which now targets main and contains this commit plus the downstream app-host cleanup commits as one landing unit. Keeping the combined PR open gives the final topology one current-main merge ref and one required CI result.