feat(launcher): drop dead frontmatter model + validate model name (never-break)

G1: remove the dead `model:` line from all 6 .openclaw/agents/*.md prompts —
launcher never read it; config (agent_model_*) is the single source of truth.

G2: add is_valid_model helper (format check ^claude-…$) applied inside
resolve_agent_model's resolution cascade and at the inline --fallback-model
read in _spawn. An invalid name is logged and skipped to the next valid level
(in the limit: no --model flag), never passed to the CLI, never raises. Format
check chosen over an allowlist for forward-compatibility (ADR-001).

G3 (routing) and G4 (fallback) intentionally NOT enabled — all agents stay on
claude-opus-4-8; agent_fallback_model stays "".

Docs (golden source) updated in the same change: README model/effort table +
validation, CLAUDE.md, .env.example (ORCH_AGENT_MODEL_*/EFFORT_*/FALLBACK_MODEL),
CHANGELOG. Tests: test_agent_frontmatter_no_model.py (G1), extended
test_resolve_agent_model.py (G2 never-break).

Refs: ORCH-074
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.openclaw/agents/developer.md

@@ -1,7 +1,6 @@
 ---
 name: developer
 description: Senior разработчик. Реализует ТЗ по ADR, пишет тесты, открывает PR.
-model: claude-sonnet-4-6
 tools:
   - Filesystem (Read везде; Write — src/, tests/, docs/work-items/*/[07-10]*, CHANGELOG.md)
   - Git (commit, push; merge запрещён)