docs: add Development section to README and update CHANGELOG

Add Development section to README with Makefile target reference
table. Fix clone URL from evansst/git-chain to dashed/git-chain.
Update CHANGELOG with all unreleased changes: error propagation
refactoring, new tests, Makefile improvements.

Author: dashed

CHANGELOG.md

@@ -7,6 +7,26 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+- Replaced `process::exit(1)` with proper error propagation in core operations
+  - `rebase`, `backup`, `push`, `prune`, and `pr` operations now return `Result<(), Error>` instead of calling `process::exit(1)`
+  - Errors propagate to the top-level handler in `main.rs` for consistent formatting
+  - `check_gh_cli_installed()` returns `Result` instead of exiting directly
+- Updated CLAUDE.md to reference Makefile targets instead of raw cargo commands
+
+### Added
+- Added `lint` Makefile target (combines `fmt-check` + `clippy-strict`)
+- Added `test-file` Makefile target for running all tests in a specific file
+- Added integration tests for error propagation:
+  - `rebase_nonexistent_chain`, `rebase_dirty_working_directory`, `rebase_missing_branch_in_chain`
+  - `backup_nonexistent_chain`, `push_nonexistent_chain`, `prune_nonexistent_chain`
+
+### Fixed
+- Updated `.PHONY` declaration in Makefile to include all targets
+
+### Removed
+- Removed dead Makefile targets: `test-bench`, `test-pr-fix`, `integration-test`
+
 ## [0.0.13] - 2025-11-05
 
 ### Improved

README.md

@@ -465,7 +465,7 @@ error: Merge conflict between feature/auth and feature/profiles
 1. Install Rust and Cargo: https://rustup.rs
 2. Get the Git Chain code:
    ```
-   git clone https://github.com/evansst/git-chain.git
+   git clone https://github.com/dashed/git-chain.git
    cd git-chain
    ```
 3. Install the tool:
@@ -483,6 +483,25 @@ Alternatively, you can create a Git alias:
 git config --global alias.chain "!/path/to/target/release/git-chain"
 ```
 
+## Development
+
+This project uses a Makefile for all development tasks. Run `make help` to see every available target.
+
+```bash
+make build                        # Build in debug mode
+make release                      # Build in release mode
+make test                         # Run all tests
+make test-sequential              # Run tests single-threaded
+make test-specific TEST=test_name # Run a specific test with output
+make test-file FILE=backup        # Run all tests in a file
+make lint                         # Format check + strict clippy
+make fmt                          # Auto-format code
+make clippy                       # Run clippy lints
+make ci-local                     # Run the full CI pipeline locally
+make clean                        # Clean build artifacts
+make debug-info                   # Show toolchain and environment info
+```
+
 ## Getting Started: A Simple Example
 
 Let's see how to use Git Chain with a simple example: