feat: model taxonomy gains gpt-5.6-sol + per-host generation defaults

Adds 'gpt-5.6-sol' to the model taxonomy with exact-match-only resolution
(Terra/Luna/suffixed IDs deliberately fall back to generic gpt) and replaces
the hardcoded 'claude' generation default with a validated
HostConfig.defaultModel: codex renders the gpt profile when --model is
absent, every other host keeps claude. Codex ship golden regenerated
accordingly; ADDING_A_HOST documents the new field.

scripts/models.ts

@@ -2,13 +2,17 @@
  * Model taxonomy — neutral module with no imports from hosts/ or resolvers/.
  *
  * Model families supported by model overlays in model-overlays/{family}.md.
- * Host configs can reference these as `defaultModel` strings (validated at
+ * Host configs reference these as `defaultModel` strings (validated at
  * generation time), but the model axis is independent of the host axis.
  *
  * IMPORTANT: host ≠ model. Claude Code can run any Claude model (Opus, Sonnet,
  * Haiku, future). Codex CLI runs GPT/o-series models. Cursor and OpenCode can
- * front multiple providers. We do NOT auto-detect the model from the host —
- * users pass --model explicitly. Default is 'claude'.
+ * front multiple providers. The generator does NOT auto-detect the model from
+ * the host — users can pass --model explicitly, otherwise each host supplies
+ * its own generation default. Exception outside this module: ./setup detects
+ * the Codex model from ${CODEX_HOME:-~/.codex}/config.toml
+ * (scripts/resolve-codex-generation-model.ts) and passes it as an explicit
+ * --model.
  */
 
 export const ALL_MODEL_NAMES = [
@@ -19,6 +23,7 @@ export const ALL_MODEL_NAMES = [
   'sonnet-5',
   'gpt',
   'gpt-5.4',
+  'gpt-5.6-sol',
   'gemini',
   'o-series',
 ] as const;
@@ -29,10 +34,11 @@ export type Model = (typeof ALL_MODEL_NAMES)[number];
  * Resolve a model argument from CLI input to a known Model family.
  *
  * Precedence rules:
- * 1. Exact match against ALL_MODEL_NAMES → return as-is.
+ * 1. Exact match against ALL_MODEL_NAMES → return as-is. This is the ONLY
+ *    path that selects `gpt-5.6-sol` — Sol is intentionally exact-only.
  * 2. Family heuristics for common variants:
  *    - `gpt-5.4-mini`, `gpt-5.4-turbo`, `gpt-5.4-*` → `gpt-5.4`
- *    - `gpt-*` (anything else GPT) → `gpt`
+ *    - `gpt-*` (anything else GPT, including other 5.6 variants) → `gpt`
  *    - `o3`, `o4`, `o4-mini`, `o1`, `o1-mini`, `o1-pro` → `o-series`
  *    - `claude-*` (sonnet, opus, haiku, any version) → `claude`
  *    - `gemini-*` (2.5-pro, flash, etc.) → `gemini`
@@ -52,6 +58,10 @@ export function resolveModel(input: string): Model | null {
   }
 
   // Family heuristics
+  // Sol never reaches here — the exact match above already returned it. Do
+  // not add a Sol family pattern: Terra, Luna, future 5.6 variants, and
+  // suffixed model IDs must NOT inherit Sol's behavioral profile; they fall
+  // through to the generic `gpt` family below.
   if (/^gpt-5\.4(-|$)/.test(s)) return 'gpt-5.4';
   if (/^gpt(-|$)/.test(s)) return 'gpt';
   if (/^o[0-9]+(-|$)/.test(s)) return 'o-series';