feat: add omairc local package

[fredimachado]

Why

Omairc is a dead-simple Qt Quick IRC client built for Omarchy (theme colors, text scale, keyboard-first). It already ships as a signed-off install path via a personal GitHub Releases pacman repo. This PR adds it to the official Omarchy Package Repository so users can install with omarchy pkg add omairc / Install > Package without a custom [omairc] Server.

Scope

• pkgbuilds/omairc/PKGBUILD: source: local build from the v0.1.0 GitHub source archive with qmake6 PREFIX=/usr and INSTALL_ROOT.
• .omarchy/package.json: source: local, release_ring: fast.
• .omarchy/upstream.sh: track newer stable vX.Y.Z GitHub releases and refresh pkgver / sha256sums.

Tradeoffs

• x86_64 only for the first cut.
• Unsigned personal Releases repo remains available as a fallback until this lands and builds.
• sha256sums is pinned to the current v0.1.0 source archive tip.

Blast Radius

• Adds one package to edge/fast builds once merged and released by maintainers.
• No changes to existing packages.

Verification

• Personal repo install already works: Server = https://github.com/fredimachado/omairc/releases/latest/download then pacman -Sy omairc.
• Upstream project: https://github.com/fredimachado/omairc
• Homepage / README install docs describe the same packaging contract (INSTALL_ROOT, Qt 6 depends).