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.

test/fixtures/golden/codex-ship-SKILL.md

@@ -113,7 +113,7 @@ if [ -d ".agents/skills/gstack" ] && [ ! -L ".agents/skills/gstack" ]; then
   fi
 fi
 echo "VENDORED_GSTACK: $_VENDORED"
-echo "MODEL_OVERLAY: claude"
+echo "MODEL_OVERLAY: gpt"
 _CHECKPOINT_MODE=$($GSTACK_BIN/gstack-config get checkpoint_mode 2>/dev/null || echo "explicit")
 _CHECKPOINT_PUSH=$($GSTACK_BIN/gstack-config get checkpoint_push 2>/dev/null || echo "false")
 echo "CHECKPOINT_MODE: $_CHECKPOINT_MODE"
@@ -579,23 +579,45 @@ At skill END before telemetry:
 ```
 
 
-## Model-Specific Behavioral Patch (claude)
+## Model-Specific Behavioral Patch (gpt)
 
-The following nudges are tuned for the claude model family. They are
+The following nudges are tuned for the gpt model family. They are
 **subordinate** to skill workflow, STOP points, AskUserQuestion gates, plan-mode
 safety, and /ship review gates. If a nudge below conflicts with skill instructions,
 the skill wins. Treat these as preferences, not rules.
 
-**Todo-list discipline.** When working through a multi-step plan, mark each task
-complete individually as you finish it. Do not batch-complete at the end. If a task
-turns out to be unnecessary, mark it skipped with a one-line reason.
+**Completion bias.** Do not end your turn with a partial solution when the full
+solution is reachable. If you encounter an error, debug it. If a test fails, fix it.
+If something is ambiguous, make your best judgment and proceed — don't stop and ask
+unless you're genuinely blocked.
 
-**Think before heavy actions.** For complex operations (refactors, migrations,
-non-trivial new features), briefly state your approach before executing. This lets
-the user course-correct cheaply instead of mid-flight.
+**Prefer doing over listing.** When you'd be tempted to write "you could also try X,
+Y, or Z," try the best option yourself. Pick, execute, report results.
 
-**Dedicated tools over Bash.** Prefer Read, Edit, Write, Glob, Grep over shell
-equivalents (cat, sed, find, grep). The dedicated tools are cheaper and clearer.
+**No preamble.** Skip "Great question!", "Let me help with that", and restating the
+user's request. Start with the work.
+
+**AskUserQuestion is NOT preamble.** The "No preamble" and "Prefer doing over listing"
+rules above do NOT apply to AskUserQuestion content. When you invoke AskUserQuestion,
+the user is about to make a decision — they need context, not terseness. Always emit
+the full format from the preamble's AskUserQuestion Format section:
+
+1. **Re-ground** (project + branch + task — 1-2 sentences).
+2. **Simplify (ELI10)** — explain what's happening in plain English a 16-year-old could
+   follow. Concrete stakes, not abstract tradeoffs. Non-negotiable; this is NOT preamble.
+3. **Recommend** — `RECOMMENDATION: Choose [X] because [one-line reason]` on its own
+   line. Never omit this line. Never collapse it into the options list.
+4. **Options** — lettered `A) B) C)` with Completeness scores (coverage-differentiated)
+   or the "options differ in kind" note (kind-differentiated).
+
+If you find yourself about to present an AskUserQuestion without the Simplify/ELI10
+paragraph, without a RECOMMENDATION line, or by just listing options and asking "which
+one?" — stop, back up, and emit the full format. The user will ask you to do it anyway,
+so do it the first time.
+
+**Reminder: subordination applies.** When a skill workflow says STOP, stop. When the
+skill asks via AskUserQuestion, that is the wait-for-user gate, not an ambiguity.
+Completion bias does not override safety gates.
 
 ## Voice
 

test/host-config.test.ts

@@ -112,6 +112,7 @@ describe('validateHostConfig', () => {
       name: 'test-host',
       displayName: 'Test Host',
       cliCommand: 'testcli',
+      defaultModel: 'claude',
       globalRoot: '.test/skills/gstack',
       localSkillRoot: '.test/skills/gstack',
       hostSubdir: '.test',
@@ -165,6 +166,12 @@ describe('validateHostConfig', () => {
     expect(validateHostConfig(c)).toEqual([]);
   });
 
+  test('invalid defaultModel is caught', () => {
+    const c = makeValid();
+    (c as any).defaultModel = 'llama-local';
+    expect(validateHostConfig(c).some(e => e.includes('defaultModel'))).toBe(true);
+  });
+
   test('invalid globalRoot is caught', () => {
     const c = makeValid();
     c.globalRoot = 'path with spaces';
@@ -470,6 +477,13 @@ describe('golden-file regression', () => {
 // ─── Individual host config correctness ─────────────────────
 
 describe('host config correctness', () => {
+  test('Codex defaults to generic GPT while all existing hosts retain Claude', () => {
+    expect(codex.defaultModel).toBe('gpt');
+    for (const host of ALL_HOST_CONFIGS.filter(h => h.name !== 'codex')) {
+      expect(host.defaultModel).toBe('claude');
+    }
+  });
+
   test('claude is the only host with real-dir-symlink strategy', () => {
     for (const config of ALL_HOST_CONFIGS) {
       if (config.name === 'claude') {