Reject missing explicit configuration files

[codeforester]

Parent: #50

Train position: 6 of 22
Phase: P0 — Trustworthy lifecycle

Problem

A user-supplied nonexistent config path currently loads as an empty mapping, allowing typos to pass silently.

Acceptance criteria

• A missing or unreadable user-supplied --config path produces an actionable usage/configuration error containing the path.
• Malformed YAML and non-mapping roots retain stable, documented errors.
• Optional implicit profile files may remain absent.
• Validation fails before runtime, log, metadata, or history side effects.
• Tests distinguish explicit paths from optional profile-discovered files.

Dependencies

• None