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/machine-output/spec.md

@@ -0,0 +1,21 @@
+## ADDED Requirements
+
+### Requirement: Can generate machine-readable output
+
+The system SHALL produce output suitable for automated scripting.
+
+#### Scenario: Install/remove count output
+- **WHEN** sync completes successfully
+- **THEN** system shall output: "<installed_count> package(s) installed, <removed_count> package(s) removed"
+
+#### Scenario: Empty output format
+- **WHEN** no packages to sync
+- **THEN** system shall output nothing (or indicate no changes)
+
+#### Scenario: Error details in output
+- **WHEN** pacman operation fails
+- **THEN** system shall include error details in output (package names, pacman error messages)
+
+#### Scenario: Exit code for scripting
+- **WHEN** sync completes
+- **THEN** system shall return exit code 0 for success, non-zero for errors