Add a cross-platform encoding guard so Windows-only CI failures are caught locally

[dgenio]

Summary

Some failures only surface on the Windows / non-default-encoding legs of the CI matrix and are fixed after merge, because the local validation commands and the lint/format/mypy CI legs all run on a single platform (ubuntu / Python 3.10). Add a lint guard for the most common cause — unqualified text I/O that relies on the platform default encoding — so these are caught locally and on the canonical lane.

Why this matters

The test matrix runs across ubuntu/windows/macos × Python 3.10–3.14 (15 combinations), but ruff check, ruff format --check, and mypy run only on ubuntu-latest / 3.10 (see .github/workflows/ci.yml lines 38–48). A contributor running the documented four commands on Linux/macOS gets a green local run, then discovers a Windows-only failure in CI. That is a slow, frustrating feedback loop and a recurring source of post-merge fix commits.

Evidence

• test(evals): read the eval report with explicit UTF-8 (Windows CI fix) — an encoding bug that passed locally and on Linux CI but broke on Windows (where the default open() encoding is not UTF-8).
• fix: make public_api_snapshot test version-aware for cross-Python CI — another "passed on the canonical lane, broke on another matrix leg" fix.
• These landed as follow-up commits rather than being caught pre-merge.

Proposed work

1. Enable a ruff rule that flags open() / Path.read_text / Path.write_text calls without an explicit encoding= (e.g. flake8-encodings-style / PLW1514 / W605-adjacent rules already available in ruff), scoped to chainweaver/ tests/ examples/.
2. Fix any existing violations surfaced by the rule.
3. Document in CONTRIBUTING.md / AGENTS.md that text I/O must pass encoding="utf-8" explicitly, with a one-line rationale (Windows default is not UTF-8).
4. Optional: run the lint/format/mypy leg on Windows as well, or add a small "encoding + cross-platform smoke" job, so the gating signal isn't single-platform.

Acceptance criteria

• A new or existing ruff rule rejects unqualified-encoding text I/O in the project source paths, and the rule runs as part of the existing blocking lint step.
• Existing violations are fixed; CI is green.
• The encoding expectation is documented next to the other code-style rules.

Notes

• Related to Ensure ruff format/lint runs as a blocking gate so formatting drift can't land on main #458 (making lint/format a reliable blocking gate). This issue narrows the gate to catch the specific class of bug that currently escapes to Windows-only CI.
• Keep the rule scoped to the same paths as the canonical commands (chainweaver/ tests/ examples/) so local and CI behavior match (lessons-learned rule on "commands that don't match CI exactly").