Document and name the overlay/pattern extra_context merge contract

[coordt]

Summary

• process_overlay merged pattern extra_context twice (before and after questions) with only a one-line comment explaining why, making the ordering invariant invisible to callers of merge_contexts.
• Extracted the second pass into a named function, render_pattern_context_after_questions, with a docstring stating the real contract: pattern extra_context may reference question answers (rendered twice), overlay extra_context may not (rendered once, before questions).
• No behavior change — verified the two merge_contexts calls are identical before/after.
• Documented that test_uses_answers_from_ui (via package_path) is the deletion test covering this invariant, rather than adding a duplicate test.

Fixes #46

Test plan

• uv run pytest tests/test_context_builder/test_overlays.py — 8 passed
• uv run pytest — 330 passed (12 pre-existing, unrelated test_git_commands.py errors from local git setup)
• ruff check / ruff format --check on changed files
• mypy on changed file — no new errors
• pre-commit hooks passed on commit

[github-actions]

PR Preview Action v1.8.1
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2026-09-27 12:44 UTC

[github-actions]

Version hint: patch
Current version: 0.9.1
New version (when merged): 0.9.2

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