fix: convert OpenCode model format to CLI slash format (#605)

* fix: convert OpenCode model format to CLI slash format

The OpenCode CLI expects models in provider/model format (e.g., opencode/big-pickle),
but after commit 4b0d1399 changed model IDs from slash format to prefix format,
the buildCliArgs() method was not updated to convert back to CLI format.

Root cause:
- Commit 4b0d1399 changed OpenCode model IDs from opencode/model to opencode-model
- The old code used stripProviderPrefix() which just removed the prefix
- This resulted in bare model names (e.g., "big-pickle") being passed to CLI
- CLI interpreted "big-pickle" as a provider ID, causing ProviderModelNotFoundError

Fix:
- Updated buildCliArgs() to properly convert model formats for CLI
- Bare model names (after prefix strip) now get opencode/ prepended
- Models with slashes (dynamic providers) pass through unchanged

Model conversion examples:
- opencode-big-pickle → (stripped to) big-pickle → opencode/big-pickle
- opencode-github-copilot/gpt-4o → (stripped to) github-copilot/gpt-4o → github-copilot/gpt-4o
- google/gemini-2.5-pro → google/gemini-2.5-pro (unchanged)

* refactor: simplify OpenCode model format conversion logic

Address review feedback from Gemini Code Assist to reduce code repetition.
The conditional logic for handling models with/without slashes is now
unified into a simpler two-step approach:
1. Strip opencode- prefix if present
2. Prepend opencode/ if no slash exists

apps/server/src/providers/opencode-provider.ts

@@ -25,7 +25,6 @@ import type {
   InstallationStatus,
   ContentBlock,
 } from '@automaker/types';
-import { stripProviderPrefix } from '@automaker/types';
 import { type SubprocessOptions, getOpenCodeAuthIndicators } from '@automaker/platform';
 import { createLogger } from '@automaker/utils';
 
@@ -328,10 +327,18 @@ export class OpencodeProvider extends CliProvider {
     args.push('--format', 'json');
 
     // Handle model selection
-    // Strip 'opencode-' prefix if present, OpenCode uses format like 'anthropic/claude-sonnet-4-5'
+    // Convert canonical prefix format (opencode-xxx) to CLI slash format (opencode/xxx)
+    // OpenCode CLI expects provider/model format (e.g., 'opencode/big-model')
     if (options.model) {
-      const model = stripProviderPrefix(options.model);
-      args.push('--model', model);
+      // Strip opencode- prefix if present, then ensure slash format
+      const model = options.model.startsWith('opencode-')
+        ? options.model.slice('opencode-'.length)
+        : options.model;
+
+      // If model has slash, it's already provider/model format; otherwise prepend opencode/
+      const cliModel = model.includes('/') ? model : `opencode/${model}`;
+
+      args.push('--model', cliModel);
     }
 
     // Note: OpenCode reads from stdin automatically when input is piped