Add dry-run flag and implement AUR support

openspec/changes/declpac-cli-tool/specs/dry-run/spec.md

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+# Dry-Run Mode
+
+## Summary
+
+Add `--dry-run` flag to simulate the sync operation without making any changes
+to the system. Shows what packages would be installed and what would be removed.
+
+## Motivation
+
+Users want to preview the effects of a sync operation before committing changes.
+This is useful for:
+- Verifying the intended changes are correct
+- Avoiding unintended package installations
+- Understanding what orphan cleanup will remove
+
+## Interface
+
+```
+declpac --dry-run --state packages.txt
+```
+
+## Behavior
+
+1. Read state files and stdin (same as normal mode)
+2. Validate packages exist (same as normal mode)
+3. Query current installed packages via `pacman -Qq`
+4. Compare declared packages to current state
+5. Identify packages that would be installed (not currently installed)
+6. Identify orphans that would be removed via `pacman -Qdtq`
+7. Output results with "Would install:" and "Would remove:" sections
+
+## Output Format
+
+```
+Installed 3 packages, removed 2 packages
+Would install: vim, git, docker
+Would remove: python2, perl-xml-parser
+```
+
+## Non-Goals
+
+- Actual package operations (no pacman -Syu, no pacman -Rns execution)
+- Package version comparison
+- Detailed dependency analysis
+
+## Trade-offs
+
+- Doesn't predict transitive dependencies that pacman might install
+- Orphan list may change after packages are installed