feat: add PR diff, PR review, and structured error handling commands

CHANGELOG.md

@@ -5,6 +5,39 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Added
+
+#### Raw API Access
+- `fgj api <endpoint>` - Make authenticated REST API requests to any Forgejo/Gitea endpoint
+  - HTTP method selection (`--method`/`-X`), auto-switches to POST when fields are provided
+  - JSON field assembly (`--field`/`-f`) with type inference (bool, int, float, null, string)
+  - Raw string fields (`--raw-field`/`-F`)
+  - Request body from file or stdin (`--input`)
+  - Custom headers (`--header`/`-H`)
+  - Path interpolation (`{owner}`, `{repo}`) from git context
+  - Response header display (`--include`/`-i`)
+
+#### Pull Request Management
+- `fgj pr diff <number>` - View the diff for a pull request
+  - Colorized output (`--color auto/always/never`)
+  - Changed file names only (`--name-only`)
+  - Diffstat summary (`--stat`)
+- `fgj pr comment <number>` - Add a comment to a pull request
+  - Body from flag (`--body`/`-b`) or file (`--body-file`, `-` for stdin)
+  - JSON output (`--json`)
+- `fgj pr review <number>` - Submit a review on a pull request
+  - Approve (`--approve`/`-a`), request changes (`--request-changes`/`-r`), or comment (`--comment`/`-c`)
+  - Body from flag or file
+  - JSON output (`--json`)
+
+#### Agentic / Machine-Readable Output
+- `--json-errors` global flag for structured JSON error output on stderr
+  - Error codes: `auth_required`, `not_found`, `api_error`, `invalid_input`, `git_detection_failed`, `network_error`
+  - HTTP status code and detail included when available
+  - Automatic mapping of API errors (401/403 → `auth_required`, 404 → `not_found`)
+
 ## [0.3.0] - 2026-03-13
 
 ### Added