Feature Request: Support updatedInput in PreToolUse hooks without requiring permissionDecision

## Summary

PreToolUse hooks currently require `permissionDecision: "allow"` to apply `updatedInput` modifications. This prevents using hooks for input sanitization while preserving the normal permission flow (Deny Rules → Allow Rules → Ask Rules → canUseTool).

## Use Case

We're building a multi-tenant Claude SDK application where we need to:
1. Pass `ANTHROPIC_API_KEY` to the SDK subprocess for API authentication
2. Prevent Bash tool commands from accessing sensitive environment variables

Our security approach:
- Intercept Bash commands via PreToolUse hook
- Prepend `unset ANTHROPIC_API_KEY FACETS_TOKEN ...;` to commands
- Let the normal permission flow decide whether to allow/deny/ask

## Current Behavior

```python
# Hook returns only updatedInput (no permissionDecision)
return {
"hookSpecificOutput": {
"hookEventName": "PreToolUse",
"updatedInput": {
"command": f"unset SENSITIVE_VAR; {original_command}"
}
}
}
```

**Result**: `updatedInput` is **ignored**, original command runs unmodified.

## Expected Behavior

Hook should be able to modify input AND continue to normal permission flow:

```
PreToolUse Hook (modify input) → Deny Rules → Allow Rules → Ask Rules → canUseTool
↓
updatedInput applied
```

## Current Workarounds (All Have Drawbacks)

| Approach | Problem |
|----------|---------|
| Hook + `permissionDecision: "allow"` | Auto-allows all Bash, bypasses user confirmation |
| `canUseTool` callback | Bypassed when Allow Rules match ("Allow for Session") |
| Don't pass API key to subprocess | SDK can't authenticate API calls |

## Proposed Solution

Support `updatedInput` without `permissionDecision`:

```python
return {
"hookSpecificOutput": {
"hookEventName": "PreToolUse",
"updatedInput": {"command": secured_command},
# No permissionDecision → continue to Deny/Allow/Ask rules with modified input
}
}
```

## Alternative Solutions

### CLI Argument for API Key Authentication

Support passing `ANTHROPIC_API_KEY` via CLI argument instead of environment variable:

```bash
claude --api-key "sk-ant-..." [other options]
```

Or via SDK options:

```python
ClaudeAgentOptions(
api_key="sk-ant-...", # Passed as CLI arg, NOT as env var
env={}, # No sensitive vars in subprocess environment
)
```

**Benefits:**
- CLI process authenticates using the argument
- Argument is NOT inherited by child processes (unlike env vars)
- Bash subprocesses never see the API key
- No hooks or input modification needed

This follows the security principle of least privilege - authentication credentials are scoped to the process that needs them, not leaked to all descendants.

## Environment

- Claude Code CLI version: 2.x
- claude-agent-sdk (Python): 0.1.6
- Platform: Linux/macOS

## Impact

This limitation affects any SDK integrator who needs to:
- Sanitize tool inputs for security
- Transform tool parameters based on context
- Protect sensitive credentials from Bash tool access
- While still respecting permission rules and user confirmations