magentic-one-cli: pin utf-8 encoding when loading YAML config (refs #5566)

[adv0r]

Dear maintainer — this PR has a permanent home with methodology + opt-out at tokens-for-good. A one-line "no thanks" → auto-close + blacklist. Sorry for the notification this edit caused.

---

Why

Refs #5566.

#5566 reported m1 crashing with
UnicodeDecodeError: 'cp950' codec can't decode byte ... on a non-UTF-8
default locale (Traditional Chinese Windows). The original call site
(playwright_controller.py) was fixed in #6094, but the reporter
flagged that there were "similar issues in the codebase while using
open function", and the issue was kept open to track the follow-up
sweep. This PR fixes the next two call sites on the same code path the
user actually hits when they type m1 ... on Windows.

What

python/packages/magentic-one-cli/src/magentic_one_cli/_m1.py loads its
YAML config in two places, both via open(..., \"r\") with no
encoding=:

Line	Source	Used for
100	DEFAULT_CONFIG_FILE	default ~/.magentic_one_config.yaml
105	args.config[…]	user-supplied --config <path>

On non-UTF-8 locales (cp950, cp1252, …) any non-ASCII byte in the
config (comments in CJK, accented paths, …) would raise
UnicodeDecodeError before the CLI gets to build an agent.

This PR pins encoding=\"utf-8\" on both, matching YAML 1.2's default
stream encoding.

- with open(DEFAULT_CONFIG_FILE, \"r\") as f:
+ with open(DEFAULT_CONFIG_FILE, \"r\", encoding=\"utf-8\") as f:

- with open(args.config if isinstance(args.config, str) else args.config[0], \"r\") as f:
+ with open(args.config if isinstance(args.config, str) else args.config[0], \"r\", encoding=\"utf-8\") as f:

Scope

Intentionally only these two call sites:

• Same package as the original crash (magentic-one-cli), same code
  path the bug report hits.
• A blanket sweep across python/packages/ would touch >40 files
  including test fixtures and benchmark scenario scripts that read
  JSONL produced by the agents themselves — forcing UTF-8 there could
  in theory mask issues. Better to land focused, then iterate.

Verification

• AST parses cleanly.
• Only two lines changed; no functional change for already-UTF-8-locale
  users.

---

AI-assisted via Cursor (Claude Opus 4.7). Personal token-burn
initiative by @adv0r to use up an expiring Cursor subscription budget on
small, useful upstream contributions.

Made with Cursor