docs: add Codex CLI competitor profile (#737)

## Summary

Adds a competitor profile for Codex CLI to the documentation.

## Changes

- Added `docs/competitors/codex.md`
- Documented Codex CLI overview, capabilities, and positioning
- Included competitor analysis and comparison details
- Updated related documentation references

## Testing

- Documentation-only change
- No code changes

Co-authored-by: Shep Bot <shep-agent@users.noreply.github.com>

Author: mayurCoder2004

docs/competitors/README.md

@@ -45,7 +45,7 @@ Idea → Requirements → Research → Plan → Code → Tests → PR → CI →
 | Parallel features via worktrees | Yes                                 | Auto-Claude, Windsurf (Wave 13)                    |
 | Open source CLI                 | Yes (MIT)                           | OpenCode, Aider, Cline, Spec Kitty                 |
 | Web dashboard                   | Yes (React Flow)                    | Auto-Claude (Electron), Spec Kitty (kanban)        |
-| Agent-agnostic                  | Claude Code, Cursor CLI, Gemini CLI | Spec Kitty (multi-agent), Gas Town (Claude only)   |
+| Agent-agnostic | Claude Code, Cursor CLI, Gemini CLI, Codex CLI | Spec Kitty (multi-agent), Gas Town (Claude only)   |
 
 ---
 
@@ -71,6 +71,7 @@ Idea → Requirements → Research → Plan → Code → Tests → PR → CI →
 | [OpenCode](./opencode.md)       | Terminal AI coding agent with 120K+ stars              | Biggest community         |
 | [Aider](./aider.md)             | AI pair programming in the terminal                    | Git-first pioneer         |
 | [Cline](./cline.md)             | VS Code agent with plan/act modes                      | 4M+ installs              |
+| [Codex](./codex.md) | OpenAI's terminal coding agent | OpenAI ecosystem |
 
 ### App Builders (Adjacent)
 

docs/competitors/codex.md

@@ -0,0 +1,79 @@
+# Codex CLI
+
+> OpenAI's terminal-first coding agent for software development workflows.
+
+|             |                                           |
+| ----------- | ----------------------------------------- |
+| **Website** | https://openai.com                        |
+| **GitHub**  | https://github.com/openai/codex           |
+| **Tagline** | OpenAI coding agent for the terminal      |
+| **Type**    | CLI coding agent                          |
+| **Pricing** | Varies by model usage                     |
+| **License** | Open source                               |
+
+---
+
+## What It Does
+
+Codex CLI is OpenAI's terminal-based coding agent. It can inspect repositories, modify files, execute commands, and assist with software development tasks directly from the command line.
+
+### Key Features
+
+- Terminal-first workflow
+- OpenAI-backed coding agent
+- Multiple GPT-5 Codex model variants
+- Repository-aware assistance
+- Command execution capabilities
+- Extensible developer workflow support
+
+---
+
+## How Shep Compares
+
+|                | Codex CLI | Shep |
+| -------------- | --------- | ---- |
+| Interface | CLI | CLI + Web dashboard |
+| Focus | Coding assistance | Full SDLC lifecycle |
+| Planning | Task-oriented | Structured planning |
+| Requirements | Not included | AI-generated PRDs |
+| CI Integration | Limited | Automatic fix loop |
+| Approval Gates | User-driven | Multi-stage approvals |
+
+### What We Respect
+
+Codex CLI represents OpenAI's investment in developer tooling and provides a lightweight terminal-first experience that integrates naturally into existing workflows.
+
+### Where Shep Differs
+
+While Codex CLI focuses on coding tasks, Shep manages the entire lifecycle from requirements gathering through planning, implementation, CI validation, and merge approval.
+
+---
+
+## Model Support
+
+Current models supported by Shep's Codex integration include:
+
+- gpt-5.4
+- gpt-5.4-mini
+- gpt-5.3-codex
+- gpt-5.3-codex-spark
+- gpt-5.2-codex
+- gpt-5.2
+
+(See `CODEX_CLI_MODELS` for the full list.)
+
+---
+
+## Sources
+
+### Product Sources
+
+- https://github.com/openai/codex
+- https://openai.com
+
+### Shep Integration Evidence
+
+- `packages/core/src/infrastructure/services/agents/common/executors/codex-cli-executor.service.ts`
+- `packages/core/src/infrastructure/services/agents/common/agent-executor-factory.service.ts`
+- `packages/core/src/infrastructure/services/tool-installer/tools/codex.json`
+- `packages/core/src/infrastructure/services/agents/sessions/codex-cli-session.repository.ts`