Add a configuration setting

The rules are in internal/config/AGENTS.md. This recipe adds a server setting (internal/config/server.go); a developer CLI setting lives in internal/config/config.go and follows the same steps.

1. Write the tests first in internal/config/: the default, the value read from DEXAFLOW_<KEY> and from LEOFLOW_<KEY>, and the values Validate refuses. env_binding_test.go and execution_test.go show the shape. Run them and watch them fail.

2. Add the field to its section struct in server.go, with a mapstructure tag and a GoDoc comment that says what it changes and what the default does.

3. Register the default in serverDefaults. Without it the environment variable never binds (TestDocumentedEnvVarsBind). Pick the default that keeps today’s behavior, so the setting is inert until someone sets it (ADR 0068).

4. Validate it. Add a check to (*ServerConfig).Validate that refuses a bad value at boot, naming the key and what it accepts.

5. Use it where it applies, and test that path through its real entry point, not only the config struct.

6. Document it. The settings tables of the configuration reference are generated from internal/config: give the key its purpose and edition in internal/tools/configref/settings.yaml (or let the field’s GoDoc describe it), make sure a BEGIN GENERATED configref settings block on the page covers it, and run make gen-config-reference. Never edit the generated rows by hand; go test ./internal/tools/configref/ fails on a stale page. Mention it on the page of the feature it tunes, and on upgrades when an operator should act. Record a changelog fragment of kind Added (record a change).

7. Expose it in the chart when operators set it through Helm: add a Helm value.

8. Check:

   go test ./internal/config/ ./internal/tools/configref/
   make lint test