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

research/agents/amp.md

@@ -415,6 +415,31 @@ if let Some(model) = options.model.as_deref() {
 3. **Wait for Amp API** — Amp may add model/mode discovery in a future release
 4. **Scrape ampcode.com** — Check if the web UI exposes available modes/models
 
+## Command Execution & Process Management
+
+### Agent Tool Execution
+
+Amp executes commands via the `Bash` tool, similar to Claude Code. Synchronous execution, blocks the agent turn. Permission rules can pre-authorize specific commands:
+
+```typescript
+{ tool: "Bash", matches: { command: "git *" }, action: "allow" }
+```
+
+### No User-Initiated Command Injection
+
+Amp does not expose any mechanism for external clients to inject command results into the agent's context. No `!` prefix equivalent, no command injection API.
+
+### Comparison
+
+| Capability | Supported? | Notes |
+|-----------|-----------|-------|
+| Agent runs commands | Yes (`Bash` tool) | Synchronous, blocks agent turn |
+| User runs commands → agent sees output | No | |
+| External API for command injection | No | |
+| Command source tracking | No | |
+| Background process management | No | Shell `&` only |
+| PTY / interactive terminal | No | |
+
 ## Notes
 
 - Amp is similar to Claude Code (same streaming format)