ci: complete Engineer closure audit

[stefan-jansen]

Summary

• add a monthly, non-blocking Python 3.15 core dependency canary linked to issue Track Python 3.15 core dependency compatibility #56
• disable persisted checkout credentials in every workflow
• quote Git tree expressions so actionlint and shellcheck pass cleanly
• confirm all 24 tracked public agent guides match current package boundaries, public APIs, and quality commands without requiring content changes
• preserve the bar-sampler hierarchy and all Original compatibility aliases

Verification

• actionlint .github/workflows/*.yml
• uv run ruff check src/ tests/ examples/ scripts/
• uv run ruff format --check src/ tests/ examples/ scripts/
• uv run ty check
• uv run pytest tests/ -q (3,925 passed, 57 deselected)
• focused API, bar hierarchy, and release-policy suite (358 passed)
• uv build
• isolated wheel installation and import ml4t.engineer
• uv run mkdocs build --strict
• pre-commit run --all-files
• ecosystem workspace and agent-guide audit

A fresh Python 3.15 stable-dependency probe still stops at the missing pydantic-core==2.46.5 wheel. Issue #56 remains open for the trigger-based wait.

Closes #55

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