Refactor of guidelines keyword into mandate

[luca-anzalone]

Description

Renames the NXS language keyword guidelines to mandate across the full stack (grammar, parser, AST nodes, runtime, internal prompts, tests, examples, and docs). The old keyword is kept as a deprecated alias for backward compatibility, which will be removed in a future version.

Type of change

• Bug fix
• New feature
• Refactoring
• Documentation
• Other (describe below)

Solution

mandate better reflects the binding, authoritative nature of the block. The rename touches every layer:

• Grammar (nxs_v2_grammar.lark): the deliberate rule now accepts (mandate | guidelines)?; both terminals and rules are defined, with guidelines annotated as deprecated. Using both in the same block is a grammar-level error.
• Parser (parser.py): guidelines and __guidelines are kept as reserved words and in the TOKEN_MAP. The guidelines() transformer delegates to mandate() after emitting a DeprecationWarning: "The 'guidelines' keyword is deprecated; use 'mandate' instead.".
• AST node (node.py): Deliberate.guidelines → Deliberate.mandate; to_nxs() always serializes as mandate: / __mandate.
• Runtime (executor.py, coder.py, script.py, expertise.py, prompt.py): all attribute accesses, local variables, string literals, and LLM prompt templates updated.
• Tests: all Python test fixtures and inline NXS strings updated; one new test (test_guidelines_deprecated_alias) verifies that old scripts still parse correctly, populate .mandate, and raise DeprecationWarning.
• NXS scripts (examples, tutorials, test scripts): guidelines: → mandate:, __guidelines → __mandate.
• Docs: language spec updated; deprecation notice added under the Mandate section.

Plugin changelogs were intentionally left unchanged as they are a historical record.

Testing

• Added or updated unit/integration tests
• All existing tests pass (pytest)

New test: test/core/test_parser.py::test_guidelines_deprecated_alias — parses a deliberate using the old guidelines: keyword and asserts:

1. .mandate is populated correctly.
2. A DeprecationWarning mentioning both guidelines and mandate is emitted.

Release Notes

The guidelines: / __guidelines keyword inside deliberate blocks has been renamed to mandate: / __mandate. Existing scripts using the old keyword continue to work but will emit a DeprecationWarning at parse time. Migrate by doing a find-and-replace of guidelines: → mandate: and __guidelines → __mandate in your .nxs files. The old keyword will be removed in a future release.