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/stdin-input/spec.md

@@ -0,0 +1,17 @@
+## ADDED Requirements
+
+### Requirement: Can read package lists from stdin
+
+The system SHALL accept package names from standard input, where each line represents a package name.
+
+#### Scenario: Packages from stdin are read correctly
+- **WHEN** package names are passed via stdin separated by whitespace, tabs, or newlines
+- **THEN** system shall parse each unique package name from the input stream
+
+#### Scenario: Empty stdin input is handled
+- **WHEN** stdin contains no package names
+- **THEN** system shall skip stdin input processing
+
+#### Scenario: Whitespace normalization
+- **WHEN** packages are separated by multiple spaces, tabs, or newlines
+- **THEN** each package name shall have leading/trailing whitespace trimmed