fix(ai): resolve engine aliases for ai({ engine })

[clawedassistant26]

The bug

ai(prompt, { engine: "cc" }) fails with moshscript: ai() needs an installed engine — try install("claude") even when Claude Code is installed and on PATH.

pickAiEngine() matched raw ENGINES keys only, so it never consulted ALIASES — while every other engine surface does resolve them via resolveEngine():

$ moshcode agents cc   →  claude agents --dangerously-skip-permissions   ✓
$ moshcode start cc    →  claude                                         ✓
$ moshcode upgrade cc  →  ✓  (README: "just one engine (name any; alias ok)")
$ ai("say hi", { engine: "cc" })  →  ✗ "needs an installed engine"

Under --dry-run it fails differently and worse. The fallback handed the raw alias to aiExecArgs(), which throws:

$ moshcode run alias.mosh --dry-run
moshscript: ai() has no headless mode for "cc"

That contradicts the intent stated on the line above it — "narrate without requiring an installed engine". A dry run should never need an engine to be present.

Repro (executed against dfcb8c1, not reasoned)

A stub claude on PATH, and alias.mosh:

const out = ai("say hi", { engine: "cc" });
say(out);

resolveEngine("cc")   -> claude      # alias resolves
isInstalled("claude") -> true        # engine is installed
pickAiEngine("cc")    -> null        # …but ai() can't see it

$ moshcode run alias.mosh            →  moshscript: ai() needs an installed engine
$ moshcode run alias.mosh --dry-run  →  moshscript: ai() has no headless mode for "cc"

The fix

• src/engines.mjs — resolve the preference through the existing resolveEngine() (which already guards against Object.prototype names, per fix(resolve): stop an Object.prototype name resolving as an engine or tool #48).
• src/cli.mjs — the dry-run fallback resolves the alias too, so narration works with nothing installed.

Deliberately unchanged:

• an unknown preference still yields null (pickAiEngine("definitely-not-an-engine"), pickAiEngine("constructor"));
• a named-but-not-installed engine still refuses to silently fall back to a different one (pickAiEngine("gemini") → null when Gemini is absent);
• no preference → the same claude, codex, opencode, gemini, aider order;
• aiExecArgs() stays strict, so a genuinely unknown engine name still throws in a dry run.

After the fix:

$ moshcode run alias.mosh            →  🧠 ai() → claude: say hi
$ moshcode run alias.mosh --dry-run  →  🧠 ai("say hi") → would run: claude -p say hi

Tests

Two added, both fail before / pass after (stash-verified with git stash push -- src/: 50 pass / 2 fail unpatched, 52/52 with the fix):

• test/engines.test.mjs — pickAiEngine resolves cc and claude-code to claude against a stub engine on PATH, and still returns null for an unknown name.
• test/cli.test.mjs — a dry-run ai() with { engine: "cc" } narrates would run: claude -p instead of throwing.

Full suite npm test: 199 tests / 199 pass / 0 fail (197 before this change).

[clawedassistant26]

Rebased on main to clear the conflict with #74. The clash was in pickAiEngine: #74 added privacycode to the default order, this PR changed the preferred branch to resolve aliases first. Resolved so both hold, alias resolution on the preferred path and privacycode kept in the default order. Full suite green after the rebase, 206 tests across 6 suites, 0 failures, and CI is passing.