Make `doctor` validate config content, not just its path

[JSONbored]

Problem: doctor only reports the config file's path today — it doesn't validate the file's actual content, so a malformed config silently degrades to defaults rather than being caught before a run.

Area: AMS / Self-host

Proposal: Add real schema validation of .gittensory-miner.yml/.json content to doctor.

Deliverables:

• doctor reports specific, actionable config errors rather than just a file path.

Acceptance criteria:

• A deliberately malformed config file is caught and clearly reported by doctor.

[linear-code]

JSO-3533