fix(docs): clarify claude_code_exec and codex_exec optimizer support

[RohithPariki]

Problem

Following the introduction of the Claude Code Exec optimizer backend (#238 / #233), there were discrepancies and omissions across the documentation regarding backend roles:

1. docs/reference/config.md marked claude_code_exec as — under the Optimizer column and omitted it from the model.optimizer_backend parameter description.
2. docs/guide/configuration.md marked both codex_exec and claude_code_exec as — under the Optimizer column, contradicting docs/reference/config.md and the actual backend implementation.
3. docs/reference/cli.md lacked examples for running training and evaluation with claude_code_exec.

Root Cause

The backend compatibility matrices and CLI reference were not completely updated when claude_code_exec optimizer support and SDK trace extraction were added in #238.

Solution

• Updated the backend tables in docs/reference/config.md and docs/guide/configuration.md to accurately indicate optimizer role support for codex_exec and claude_code_exec.
• Updated the model.optimizer_backend description in docs/reference/config.md to include claude_code_exec.
• Added explicit training and evaluation CLI examples for claude_code_exec in docs/reference/cli.md, along with trace injection notes.

Testing

• Verified Markdown formatting and syntax.
• Cross-referenced the documentation against backend dispatch logic in:
  • skillopt/model/init.py
  • skillopt/model/backend_config.py
• Cross-referenced against the relevant test cases:
  • tests/test_role_backend_resolution.py
  • tests/test_claude_code_exec_resolution.py

Risk

Low risk. Documentation-only update.

Issue

Closes #268