Publish Linux desktop archives through the release pipeline

[steipete]

Published GitHub releases currently ship CLI archives while the Linux desktop is only a CI artifact. This adds a release workflow that builds/tests native x86_64 and ARM64 desktop archives from the release tag and uploads both architectures plus SHA-256 checksums only after both builds pass.

CI and releases share the same composite build action. The release version is compiled into the app, asset names follow CodexBarDesktop-v-linux-.tar.gz, and the existing mac-release asset wait/check contract now requires all four desktop assets. Manual workflow dispatch produces preview artifacts without publishing. Updated release and installation documentation covers dependencies, separate CLI downloads, and upgrades.

Validation:

• Qt build and --version check with 0.0.0-release-test: passed.
• All 14 isolated desktop runtime tests: passed.
• Package/install, asset-name contract, invalid-version, and checksum tests: passed.
• YAML parsing, actionlint, and git diff --check: passed.
• Required make test cannot start without Swift; make check stops at missing macOS plutil on this host. No Swift sources changed.

No release was created or backfilled. Release binaries target Ubuntu 24.04-compatible Qt/glibc; older systems can build from source.

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