fix(config): harden model validation and config type-checking (#128)

[jasonssdev]

Summary

Slice A of #128. Fixes the config-corruption cascade a real first-time user hit on published 0.1.1 by answering yes to the free-text model prompt in openkos init — model: yes round-tripped through PyYAML as the boolean True, silently corrupting openkos.yaml, crashing openkos doctor, and making every LLM call fail with an opaque 400.

Three independent hardening defects, all closed at the config layer:

1. validate_model rejects YAML 1.1 reserved words — yes/no/true/false/on/off/null (case-insensitive), via an exact-token frozenset check applied before the character allowlist. Tags containing a reserved substring (yesmodel, on-prem) are still accepted.
2. read_config type-checks model and embedding_model — a present non-str value raises ValueError with actionable messaging; null/absent still falls back to defaults, and unrelated bool fields (e.g. review: false) are untouched.
3. doctor no longer crashes on a non-str model. The read_config guard fully subsumes the TypeError at model_tag_matches (verified by control-flow trace: a raising read_config leaves cfg = None, so the checks fall back to valid default tags). No production change to main.py/ollama.py — only a regression test.

Scope

Production change is a single file: src/openkos/config.py (+24). Tests: test_config.py (+98), test_doctor.py (+52).

Verification

• uv run pytest: 2089 passed, exit 0
• uv run ruff check . && uv run ruff format --check .: clean
• uv run mypy .: clean
• SDD verify: PASS — 4/4 requirements, 15/15 scenarios
• Adversarial reliability review: 0 findings (reserved-word set confirmed to match PyYAML's default resolver exactly; defect feat(cli): add openkos init — create an OKF workspace #3 subsumption confirmed)

Strict TDD throughout (RED confirmed for the right reason before each GREEN, including a git stash to prove the doctor TypeError pre-fix).

Issue

Advances #128 (Slice A: config hardening). Does not close it — the interactive model picker (Slice B) remains as a follow-up.