Priority 2 improvements:
- Extract hardcoded values to src/constants.rs (API endpoints, headers, defaults)
- Replace std::fs with tokio::fs for non-blocking file operations
- Replace std::process with tokio::process for async git commands
- Add exponential backoff retry logic for all GitHub API calls (max 3 retries)
- Update cli.rs to use constants for default values
- All 27 tests passing, clippy clean

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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

…un, JSON output, and GitHub Enterprise support

This commit implements all Priority 3 enhancements from the improvement roadmap:

**New Features:**

1. Advanced Filtering Options (3.1)
   - Created filters module with support for glob patterns, regex, and exclusion
   - Smart pattern detection (regex → glob → substring priority)
   - Multiple filters with AND logic (comma-separated)
   - Examples: *.deb, linux-.*-amd64, !windows

2. JSON Output Format (3.2)
   - Added --format flag with table (default) and json options
   - Added Serialize trait to all data models
   - Machine-readable output for scripting and automation

3. Response Caching (3.3)
   - Created cache module with file-based caching (~/.cache/ghr/)
   - 24-hour TTL with automatic expiration
   - Added --cache flag to enable caching
   - Reduces API calls and improves performance

4. Dry-Run Mode (3.4)
   - Added --dry-run flag for preview mode
   - Shows what would be downloaded/cloned without executing
   - Displays asset sizes, total size, and destinations

5. GitHub Enterprise Support (3.5)
   - Added --api-url flag for custom API base URLs
   - Created _with_base variants for all endpoint builders
   - Created _with_cache variants for all GitHub API functions

**Implementation Details:**
- Added dependencies: regex (1.10), globset (0.4)
- Added error types: RegexError, GlobError, JsonError
- Created 2 new modules: filters.rs (122 lines), cache.rs (147 lines)
- Updated 9 files: cli.rs, constants.rs, errors.rs, github.rs, main.rs, models.rs, etc.
- Added 9 new tests (36 total tests, 100% pass rate)
- Clippy clean (only dead_code warnings for wrapper functions)

**Documentation:**
- Completely updated README.md with new feature documentation
- Added comprehensive examples for all new features
- Enhanced CI/CD pipeline examples
- Updated common use cases with new capabilities

**Files Changed:**
- New: src/cache.rs, src/filters.rs
- Modified: Cargo.toml, README.md, src/cli.rs, src/constants.rs,
  src/errors.rs, src/github.rs, src/main.rs, src/models.rs
- Total: +805 insertions, -98 deletions

All tests passing. Ready for release.

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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>