fix(acp): complete extension-only slash commands

[depoll]

Summary

• complete a successful Pi extension command from its RPC acknowledgement when it did not start an agent run
• keep waiting for agent_settled when an extension command does start a run
• distinguish exact extension commands from prompt and skill commands through get_commands

Pi extension commands such as /wt create can finish entirely inside their command handler. Pi emits no agent_settled event in that path, so the ACP session/prompt request previously stayed open forever and every later prompt queued behind it.

Fixes #84.

Validation

• npm run typecheck
• npm run lint
• npm test (99 passed)
• npm run build
• raw ACP probe against real Pi + pi-worktrees: /wt create native-probe returned end_turn, created the managed worktree, and switched Pi into its inherited worktree session