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/orphan-cleanup/spec.md

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+## ADDED Requirements
+
+### Requirement: Can mark non-state packages as non-explicit
+
+The system SHALL mark all packages not declared in the state as non-explicit
+(dependencies) before syncing, so they can be safely removed later.
+
+#### Scenario: Mark packages as non-explicit
+- **WHEN** packages are declared in state
+- **THEN** system shall mark those packages as explicitly installed via pacman -D --explicit
+- **AND** mark all other installed packages as non-explicit
+
+### Requirement: Can clean up orphaned packages
+
+After syncing, the system SHALL remove packages that are no longer required
+(as dependencies of removed packages).
+
+#### Scenario: Orphan cleanup after sync
+- **WHEN** sync operation completes successfully
+- **THEN** system shall run pacman -Rsu to remove unneeded dependencies
+- **AND** report the number of packages removed
+
+#### Scenario: Orphan cleanup respects explicitly installed
+- **WHEN** a package not in state is marked as explicitly installed by user
+- **THEN** system shall NOT remove it during orphan cleanup
+
+#### Scenario: No orphans to clean
+- **WHEN** there are no orphaned packages to remove
+- **THEN** system shall report "No packages to remove" in output