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/pacman-sync/spec.md

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+## ADDED Requirements
+
+### Requirement: Can sync system with declared package state
+
+The system SHALL execute pacman operations to install and upgrade all declared packages.
+
+#### Scenario: Full system upgrade with all packages
+- **WHEN** system has declared package list
+- **THEN** system shall run pacman -Syu with all declared package names
+
+#### Scenario: No partial upgrades
+- **WHEN** running pacman commands
+- **THEN** system shall use -Syu flag (full system upgrade) ensuring all packages are latest
+
+#### Scenario: Package availability check
+- **WHEN** a package from input is not in pacman repositories
+- **THEN** system shall report the error and include package name
+
+#### Scenario: Pacman execution capture
+- **WHEN** pacman commands execute
+- **THEN** system shall capture both stdout and stderr for error reporting
+
+#### Scenario: Exit code propagation
+- **WHEN** pacman commands execute
+- **THEN** system shall exit with code equal to pacman's exit code for success/failure detection