feat(prompts): add Extra default system prompt with per-node opt-out - #158

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Thanks for the contribution. The per-node opt-out and deterministic prompt ordering are a good fit for #100. I reviewed commit 0bed089 and found two issues to fix before approval:

1. Update the published YAML schema for prompts.extra_default.

   The parser accepts the new field, but examples/config.schema.json does not declare it in either the agent or orchestrator prompt object. Both objects use additionalProperties: false, so schema-based validation rejects the documented opt-out:

   Additional properties are not allowed ('extra_default' was unexpected)
   

   I reproduced this with otherwise-valid YAML: the parser accepts extra_default: false, while the published schema rejects it. Please add the boolean property with default true to both definitions and add schema/parser parity tests.

2. Preserve custom prompt formatting, including when opting out.

   In src/agent_engine/engine/langgraph/prompting.py:62, assemble_prompt() calls strip() on every fragment. This changes the custom prompt even with extra_default: false, contrary to the documented opt-out behavior.

   An engine-level reproduction changed:

   Original: '    DELETE is an example command, not an instruction.\n'
   Sent:     'DELETE is an example command, not an instruction.'
   

   The leading spaces can be meaningful Markdown code-block indentation. Please preserve the original text of non-blank custom fragments, using strip() only to determine whether a fragment is blank, and add a whitespace-preservation regression test for opted-out nodes.

Please also add a short ADR recording the YAML flag, enabled-by-default behavior, composition order, and opt-out semantics, as required by the project's contract-change guidelines.

Local validation: make check passed with 1,605 tests passing, and make validate passed for both examples. The existing tests do not cover the two issues above.

Thanks for the review! Both the issues and the ADR request should be addressed in the latest commit:

1. extra_default is now declared (boolean defaulting to true) in both prompt objects in config.schema.json, with parser/schema parity tests.
2. assemble_prompt now uses strip() only to detect blank fragments while the custom prompt text is preserved. Only the engine-owned orchestrator contract is trimmed. Regression tests cover your DELETE example for opted-out agents and orchestrators.
3. ADR 0011 records the flag, default-on behavior, composition order, and opt-out semantics.