fix: drop --gemini-model on hard-coded codex/claude lines (#130, v2.1.15)

`injectConfigVariables()` previously did a global regex replace of
`{{GEMINI_MODEL_FLAG}}`, emitting `--gemini-model` on every line — even
lines that hard-code `--backend codex` (backend.md / codex-exec.md).
codeagent-wrapper warns and ignores it, but the dead flag was
misleading and looked like a real bug to users.

Fix: line-aware substitution. After BACKEND_PRIMARY / FRONTEND_PRIMARY
have been substituted, scan each line with the flag placeholder:
- Hard-coded non-gemini backend (`--backend codex`, `--backend claude`)
  → strip the flag
- Conditional (`--backend <codex|gemini>`) or gemini hard-coded
  → keep the flag (AI picks backend at runtime)

+11 unit tests covering codex-only / gemini-only / conditional /
frontend-codex-backend-gemini / no-gemini / default model / custom
model / real-template integration. Total 150/150 passing.

Also bumps bottom README version badge 2.1.11 → 2.1.15.