Add CLI documentation to the Overview section

[codegen-sh]

This PR adds a new CLI documentation section to the Overview in the docs. The CLI section is placed after the API section as requested.

The documentation includes:

• Installation instructions
• Authentication methods
• Basic usage examples
• Available commands with descriptions
• Common workflows
• Environment variables
• Use cases and benefits

This provides a good starting point for users to understand and use the Codegen CLI.

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💻 View my work • 👤 Initiated by @rushilpatel0 • About Codegen
⛔ Remove Codegen from PR • 🚫 Ban action checks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

[codegen-sh]

Found 1 critical and 1 important issue. PR is already merged/closed, so I couldn’t add inline comments—putting the actionable suggestions here.

1. Security vulnerability: Token exposure via CLI flag (CRITICAL)
   Passing secrets via command-line flags leaks into shell history, process lists (ps/top), and sometimes telemetry. Recommend documenting safer alternatives and de-emphasizing the --token flag.

### `codegen login`

**Description:** Store authentication token.

**Usage Patterns:**
- **Interactive login (recommended):** `codegen login`
- **From environment variable (recommended):** `CODEGEN_API_TOKEN=YOUR_API_TOKEN codegen login`
- **From stdin (safer than flag):** `echo "$CODEGEN_API_TOKEN" | codegen login`

**Options:**
- `--token TEXT` API token to store (not recommended; exposes token in shell history and process list)
- `--no-verify` Skip token verification (flag)

2. Logic/Docs error: Incorrect env var name for --org-id default (IMPORTANT)
   In the codegen agent section, the --org-id option says it defaults to CODEGEN_API_TOKEN/REPOSITORY_ORG_ID. This is misleading: API token is not an organization ID. Other sections use CODEGEN_ORG_ID, which looks correct.

**Options:**
- `--prompt, -p TEXT` The prompt to send to the agent (mutually exclusive with --id unless using create)
- `--id INT` Agent run ID to fetch or pull
- `--json` Output raw JSON response when fetching (flag)
- `--org-id INT` Organization ID (defaults to CODEGEN_ORG_ID/REPOSITORY_ORG_ID or auto-detect)
- `--model TEXT` Model to use for this agent run (optional)
- `--repo-id INT` Repository ID to use for this agent run (optional)

Please review these and update the docs to avoid unsafe guidance and confusion around env vars.

🔍 View my analysis: https://codegen.com/agent/trace/77751

[github-actions]

🎉 This PR is included in version 0.56.14 🎉

The release is available on GitHub release

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