Replace per-setting configuration files with config.yaml #38

Problem

Project and user configuration currently use one file per setting under .code-converge/ (for example mode, review-model, and max-cycles). This is difficult to discover, review, and maintain as the configuration surface grows.

Desired outcome

Use one YAML configuration document named config.yaml in each supported .code-converge directory:

• <git-root>/.code-converge/config.yaml for project settings;
• ~/.code-converge/config.yaml for user settings.

Keep the existing source precedence: CLI flags > project YAML > user YAML > environment > built-in defaults.

Acceptance criteria

• config.yaml is the only supported file-based settings source in both project and user .code-converge directories.
• It represents every setting currently configured through per-setting value files, including mode, workflow options, stage-specific model/reasoning overrides, review base, and diagnostic session-log settings.
• The documented YAML schema is typed, validated, and rejects unknown keys and invalid values with actionable diagnostics.
• code-converge config reports effective values and their existing sources correctly when values come from YAML.
• Existing CLI flags and CODE_CONVERGE_* environment variables retain their current names and precedence.
• Legacy per-setting files are not read and no migration, fallback, or compatibility behavior is implemented.
• Decide and document the YAML representation for the three prompt settings; a file reference is acceptable if prompt contents should remain separate from configuration.
• Update README and the public CLI/configuration contract with complete project and user examples.
• Add focused precedence, validation, malformed-YAML, unknown-key, and no-legacy-fallback tests.

Scope notes

This is a clean-break change to the public configuration contract and belongs to Feature Flow, not Small Change. It should include a versioning/release decision appropriate for the breaking configuration change.