Add linux-amd64 support (install + release artifacts)

[jeffrichley]

What

Adds linux/amd64 as a supported install/run target for looper and looperd.

The daemon's default mode is already foreground/detached (cross-platform Go), so this change only:

1. CLI target resolver (resolveLooperdTarget) accepts linux-amd64.
2. Install script (scripts/install.sh) detects Linux/x86_64.
3. Release workflow publishes looper-linux-amd64 / looperd-linux-amd64, built on an ubuntu-latest runner so the post-build --version self-check runs the binary natively.
4. Docs updated for Linux.

Out of scope (follow-up)

Supervised lifecycle on Linux via a systemd daemon mode (parity with macOS launchd). On Linux today the daemon runs in foreground mode; --daemon-mode launchd stays macOS-only and returns its existing actionable error.

Testing

• go test ./internal/cliapp/ passes, including the updated TestResolveLooperdTarget (now asserts linux-amd64 resolves).
• scripts/install.sh detect_target verified to emit linux-amd64 on Linux/x86_64, darwin-arm64 on macOS/arm64, and fail cleanly on unsupported arches.
• Built both binaries on linux/amd64 (WSL2) and confirmed end to end:
  • looper daemon install --json → "target": "linux-amd64"
  • looper daemon start → daemon starts detached
  • looper status → healthy: yes, daemonMode: foreground, DB migrations applied, scheduler healthy
  • looper daemon stop → stops cleanly

Caveat

The release-workflow matrix additions could not be exercised in upstream CI from a fork; they mirror the existing darwin-arm64 rows exactly, on an ubuntu-latest runner.

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