Add OpenSpec for declpac CLI tool

- OpenSpec for declarative pacman package management
- stdin and --state file inputs with merging
- Full system upgrades, transitive deps, orphan cleanup
- AUR support with pacman/makepkg fallback
- Machine-readable output for scripting

openspec/changes/declpac-cli-tool/specs/aur-sync/spec.md

@@ -0,0 +1,23 @@
+## ADDED Requirements
+
+### Requirement: Can handle AUR packages with pacman fallback
+
+The system SHALL attempt to install AUR packages by first trying pacman
+repositories, then falling back to AUR when pacman fails, and finally reporting
+errors for packages still missing.
+
+#### Scenario: Install from pacman first
+- **WHEN** package is in pacman repositories
+- **THEN** system shall install via pacman
+
+#### Scenario: Fall back to AUR
+- **WHEN** package is not in pacman repositories but is in AUR
+- **THEN** system shall attempt installation via makepkg (direct AUR build)
+
+#### Scenario: Report error for missing packages
+- **WHEN** package is not in pacman repositories or AUR
+- **THEN** system shall report error and print to stderr
+
+#### Scenario: Continue for remaining packages
+- **WHEN** some packages are installed and some fail
+- **THEN** system shall continue installation process for successful packages