acp: IDE-proxied terminal execution

[bug-ops]

Parent: #762

Task

Proxy shell command execution through the IDE client using ACP terminal methods instead of direct subprocess spawning.

Acceptance criteria

• terminal.rs — AcpTerminal struct for IDE-proxied command execution
• terminal/create → create terminal in IDE with specified command
• terminal/output → poll terminal output
• terminal/wait_for_exit → await command completion
• terminal/kill → terminate running command
• terminal/release → release terminal resources
• Adapter implementing ToolExecutor to replace ShellExecutor in ACP mode
• Timeout handling for long-running commands