fix: credential detection and provider auth status (#120)

## Summary

Fix credential detection bugs and add credential availability status to the API. Consolidate Claude fallback models and add `sonnet` alias.

Builds on #109 (OAuth token support).

Related issues:
- Fixes #117 (Claude, Codex not showing up in gigacode)
- Related to #113 (Default agent should be Claude Code)

## Changes

### Credential detection fixes
- **`agent-credentials/src/lib.rs`**: Fix `?` operator bug in `extract_claude_credentials` - now continues to next config path if one is missing instead of returning early

### API credential status
- **`sandbox-agent/src/router.rs`**: Add `credentialsAvailable` field to `AgentInfo` struct
- **`/v1/agents`** endpoint now reports whether each agent has valid credentials

### OpenCode provider improvements
- **`sandbox-agent/src/opencode_compat.rs`**: Build `connected` array based on actual credential availability, not just model presence
- Check provider-specific credentials for OpenCode groups (e.g., `opencode:anthropic` only connected if Anthropic creds available)
- Add logging when credential extraction fails in model cache building

### Fallback model consolidation
- Renamed `claude_oauth_fallback_models()` → `claude_fallback_models()` (used for all fallback cases, not just OAuth)
- Added `sonnet` to fallback models (confirmed working via headless CLI test)
- Added `codex_fallback_models()` for Codex when credentials missing
- Added comment explaining aliases work for both API and OAuth users

### Documentation
- **`docs/credentials.mdx`**: New reference doc covering credential sources, extraction behavior, and error handling
- Documents that extraction failures are silent (not errors)
- Documents that agents spawn without credential pre-validation

### Inspector UI
- **`AgentsTab.tsx`**: Added credential status pill showing "Authenticated" or "No Credentials"

## Error Handling Philosophy

- **Extraction failures are silent**: Missing/malformed config files don't error, just continue to next source
- **Agents spawn without credential validation**: No pre-flight auth check; agent's native error surfaces if credentials are missing
- **Fallback models for UI**: When credentials missing, show alias-based models so users can still configure sessions

## Validation

- Tested Claude Code model aliases via headless CLI:
  - `claude --model default --print "say hi"` ✓
  - `claude --model sonnet --print "say hi"` ✓
  - `claude --model haiku --print "say hi"` ✓
- Build passes
- TypeScript types regenerated with `credentialsAvailable` field

frontend/packages/inspector/src/components/debug/AgentsTab.tsx

@@ -39,6 +39,7 @@ const AgentsTab = ({
         : defaultAgents.map((id) => ({
             id,
             installed: false,
+            credentialsAvailable: false,
             version: undefined,
             path: undefined,
             capabilities: emptyFeatureCoverage
@@ -49,6 +50,9 @@ const AgentsTab = ({
             <span className={`pill ${agent.installed ? "success" : "danger"}`}>
               {agent.installed ? "Installed" : "Missing"}
             </span>
+            <span className={`pill ${agent.credentialsAvailable ? "success" : "warning"}`}>
+              {agent.credentialsAvailable ? "Authenticated" : "No Credentials"}
+            </span>
           </div>
           <div className="card-meta">
             {agent.version ? `v${agent.version}` : "Version unknown"}