chore: open files with encoding=utf-8

[SoundMatt]

Three open() calls in the spec_history modules read or wrote files without specifying encoding="utf-8", falling back to Python's locale-dependent default. On Windows or systems whose default locale isn't UTF-8, GraphQL type definitions and JSON-LD spec history files containing non-ASCII characters in descriptions or comments would be silently corrupted (write side) or fail to parse (read side).

Sites fixed:

• src/s2dm/exporters/spec_history.py:247 — type-definition write
• src/s2dm/registry/spec_history.py:71 — JSON load
• src/s2dm/registry/spec_history.py:80 — JSON-LD save

The other open() calls in the codebase (vspec.py, cli.py, concept_uri.py, naming_config.py, variant_ids.py) already pass encoding="utf-8" correctly. This brings the spec_history modules in line.

No behaviour change on macOS/Linux where the default locale is already UTF-8. Local verification: 532 tests pass.

[cpodalak]

@SoundMatt, checks faiiling.. ⬆️

[SoundMatt]

@cpodalak — the CI hasn't run yet because this is a fork PR and needs a maintainer to click "Approve and run" on the Actions tab to trigger the workflows. The commit 8014cef has the correct fix: conventional-commit prefix so gitlint should pass this time. Could you approve the CI run?