feat: discover external SOPs recursively - #78

Issue #, if available:

Closes #61

Description of changes:

• Discover external .sop.md files recursively under every configured SOP path.
• Sort matches by their relative POSIX path so same-name first-wins precedence is deterministic across platforms.
• Add regression coverage for discovery order and the MCP, Agent Skills, and Cursor command consumers.
• Document recursive discovery in the external SOP loading guides.

How to verify:

• MCP 1.x test axis on Windows: 46 passed, 6 deselected.
• MCP 2.x test axis on Windows: 46 passed, 6 deselected.
• The six deselected cases are existing POSIX path-separator expectations that also fail on unchanged main; the affected nested-discovery tests pass in both axes.
• ruff check .
• uv build
• git diff --check
• Mutation check: replacing recursive discovery with the previous flat glob makes all four nested-discovery regressions fail.

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