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/input-merging/spec.md

@@ -0,0 +1,24 @@
+## ADDED Requirements
+
+### Requirement: Can merge multiple input sources
+
+The system SHALL combine package lists from all inputs using an additive
+strategy without conflict resolution.
+
+#### Scenario: Additive merging of all inputs
+- **WHEN** stdin and multiple state files provide package lists
+- **THEN** system shall include all packages from all inputs in the final state
+
+#### Scenario: Input priority is last-writer-wins per file
+- **WHEN** multiple state files contain the same package name
+- **THEN** the package from the last provided file in command-line order takes
+  precedence
+
+#### Scenario: Missing packages from stdin are added
+- **WHEN** stdin contains packages not in state files
+- **THEN** those packages shall be added to the final state
+
+#### Scenario: Duplicate packages accumulate
+- **WHEN** the same package appears in multiple inputs
+- **THEN** it shall be included multiple times in the final state (pacman
+  handles duplicates)