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/transitive-deps/spec.md

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+## ADDED Requirements
+
+### Requirement: Can resolve transitive dependencies automatically
+
+The system SHALL automatically determine and install all transitive dependencies
+for declared packages, so users only need to specify direct packages.
+
+#### Scenario: Transitive dependencies are resolved
+- **WHEN** a package with dependencies is declared in state
+- **THEN** system shall identify all transitive dependencies via pacman
+- **AND** include them in the installation operation
+
+#### Scenario: Dependency resolution includes AUR dependencies
+- **WHEN** a package's transitive dependencies include AUR packages
+- **THEN** system shall resolve those AUR dependencies via makepkg
+
+#### Scenario: Shared dependencies are deduplicated
+- **WHEN** multiple declared packages share dependencies
+- **THEN** system shall install each shared dependency only once
+
+#### Scenario: Missing dependency handling
+- **WHEN** a declared package has a dependency not available in pacman or AUR
+- **THEN** system shall report error for the missing dependency