Fix seam-doctor crash under a non-UTF-8 locale

[justin808]

Problem

agent-workflow-seam-doctor is the validation AGENTS.md mandates before any
issue / PR / batch work. It crashes under a non-UTF-8 locale:

$ LANG=C LC_ALL=C ruby bin/agent-workflow-seam-doctor --root <repo>
agent-workflow-seam-doctor:122:in 'String#match?': invalid byte sequence in US-ASCII (ArgumentError)

LANG=C / LC_ALL=C is common in CI and headless agent shells. It passes
interactively only because dev shells default to en_US.UTF-8.

Root cause

check read AGENTS.md with File.read(agents_path), which decodes using the
locale's default external encoding. Under a non-UTF-8 locale the string is
tagged US-ASCII, so the first non-ASCII byte (em dash, arrow, …) makes
String#match? raise while extract_section scans the
## Agent Workflow Configuration heading. Every adopting repo is affected,
because real AGENTS.md files contain such bytes.

Fix

Read AGENTS.md as UTF-8 and scrub, the same idiom already used for
skill/workflow markdown in executable_placeholder_issues
(File.binread(path).force_encoding("UTF-8").scrub). One line, no behavior
change under a UTF-8 locale.

Testing

• New regression test test_non_ascii_agents_md_parses_under_ascii_locale
  writes a non-ASCII AGENTS.md and runs the doctor with LC_ALL=C / LANG=C;
  it fails (the crash) before the fix and passes after.
• bin/validate is green (40 runs, 101 assertions, 0 failures).
• Verified against a real consumer repo: LANG=C LC_ALL=C ... --root <react_on_rails>
  now returns PASS instead of crashing.

Note

Other standalone Ruby helpers in skills/*/bin/ use File.read too and may
warrant the same hardening; left out here to keep this fix focused.

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 87ac6df4-1f2b-4cda-873f-bbd27519b69a

📥 Commits

Reviewing files that changed from the base of the PR and between d38645b and cb71a54.

📒 Files selected for processing (2)

• bin/agent-workflow-seam-doctor
• bin/agent-workflow-seam-doctor-test.rb

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch jg/seam-doctor-utf8-locale-crash

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