docs: add capability protocol mini demo

[snoopuppy582]

Fixes #275

Summary

• Added a 71-line runnable capability protocol mini-demo that registers in-process predictor, policy, and cost objects.
• Added a focused quickstart page contrasting protocol registration with the BaseProvider path.
• Added a subprocess test that runs the example and asserts the emitted plan JSON.
• Linked the new page from the Python API docs, provider authoring guide, SUMMARY, mkdocs nav, and CHANGELOG.

Tests

• uv run python examples/capability_protocols_mini.py
• uv run pytest tests/test_capability_protocols_mini_example.py tests/test_capability_protocols.py -q
• uv run pytest tests/test_docs_site.py -q
• uv run ruff check examples/capability_protocols_mini.py tests/test_capability_protocols_mini_example.py
• uv run python scripts/check_docs_snippets.py
• uv run mkdocs build --strict
• git diff --check

Notes

• uv run python scripts/check_docs_commands.py still fails on existing stale script references across AGENTS/README/docs; I did not change that surface in this PR.

[AbdelStark]

please resolve conflicts

[snoopuppy582]

Thanks for the heads up. I rebased this branch onto the latest main after #291 merged and resolved the CHANGELOG.md conflict, keeping both changelog entries.

Re-ran locally:

• uv run pytest tests/test_capability_protocols_mini_example.py tests/test_capability_protocols.py -q -> 20 passed
• uv run pytest tests/test_docs_site.py -q -> 60 passed
• uv run ruff check examples/capability_protocols_mini.py tests/test_capability_protocols_mini_example.py -> passed
• uv run ruff format --check examples/capability_protocols_mini.py tests/test_capability_protocols_mini_example.py -> passed
• uv run python scripts/check_docs_snippets.py -> passed
• uv run mkdocs build --strict -> passed
• git diff --check -> passed