ci: fail PRs when the release toolchain cannot build the app

[FZ2000]

Problem

PR #190 passed every CI check yet main could not be built into a signed DMG on the release machine (reported: macOS 15.7 / Xcode 26.3, swift build -c release failing with ambiguous use of operator '-' in WorkTimeCanvas.swift). #212 carries the three-line fix.

Change

Adds macos-release-build: macos-15 (the release OS) with Xcode_26.3 (the release toolchain), running swift build -c release in apps/agentacct — exactly build-app.sh's compile command.

Reproduction status: not yet achieved (full matrix below)

Building the broken pre-fix revision (5aff13a) succeeds on everything reachable:

Configuration	Result on pre-fix code
Xcode 26.6 (runner default)	builds
Xcode 26.3 on macos-26	builds
Xcode 26.3 on macos-15 (log verified "Xcode 26.3", 17C529)	builds
Xcode 26.2 / 26.1.1 / 26.0.1 on macos-15	builds (CI matrix)
Xcode 16.4	builds
Swift main development snapshot (preview), typecheck of the exact expression	builds
Swift 5 and Swift 6 language modes, arm64 and x86_64	builds
Type-check cost of the expression	~2 ms (no timeout risk)

Searches of local transcripts, the agentacct ledger and the Swift issue tracker found no matching diagnostic or documented regression. Conclusion: the release-only failure is not reproducible with any toolchain, SDK or language configuration available here; it is specific to that machine's environment.

What is needed to make this leg a true reproducer

Run on the release machine and paste the output:

xcode-select -p
xcodebuild -version
swift --version
cd <checkout> && swift build -c release 2>&1 | tee /tmp/release-build.log

The first error block from /tmp/release-build.log plus those three version lines will identify the toolchain/state that fails (candidate explanations: a beta/regressed Swift build behind "26.3", a CommandLineTools-vs-Xcode mismatch, or stale incremental state). If it is a swift.org or beta toolchain, this job will pin that exact version.

Until then this check is release-OS/toolchain parity and defense-in-depth; it is not claimed to catch the #212 class.

[FZ2000]

Closing without merging. The failure this job guarded against (ambiguous use of operator '-' in WorkTimeCanvas.swift on the release machine) could not be reproduced anywhere: Xcode 26.0.1-26.6 on macos-15/macos-26, Xcode 16.4, Swift 5/6 modes, a Swift main snapshot, and x86_64 cross-builds all compile the pre-fix revision (5aff13a) cleanly. No local session, transcript, or ledger records the diagnostic either. Per Frank: not reproducible here, so not a bug we should fix. The clarifying fix (#212) is already merged and harmless. Reopen if a release-machine build log ever surfaces.