Restore bounded project context overview - #972

Summary

• Restore initial project awareness by adding a bounded file overview to project context.
• Prefer git ls-files for repo-aware, ignored-file-safe listing, with a capped directory-scan fallback outside git repos.
• Document that project context includes the file overview and add regression coverage for git, truncation, and fallback paths.

Verification

• python -m py_compile vibe/core/system_prompt.py vibe/core/config/models.py tests/core/test_system_prompt.py
• git diff --check
• Commit/push hooks: ruff check, ruff format, typos passed
• Blocked: uv run pytest tests/core/test_system_prompt.py tests/test_system_prompt.py -q and uv run ruff ... because uv could not fetch pinned build dependencies from https://pypi.fury.io/mistralai/ (tunnel error: unsuccessful)
• Skipped pyright hook for commit/push because the incomplete uv environment could not resolve pytest