Add ACP support for /goal and /subgoal

Problem

Hermes already supports standing goals through the existing gateway and CLI GoalManager flow, but ACP clients such as Zed do not currently get that same workflow.

ACP currently advertises local slash commands such as /steer and /queue, but it does not advertise or handle /goal or /subgoal. As a result, users connected through Zed/ACP cannot set a standing goal, manage subgoals, or rely on Hermes to continue work across turns the way gateway and CLI users can.

Repro

1. Start the ACP adapter, for example with hermes acp.
2. Connect from Zed or another ACP client.
3. Inspect available slash commands or type /goal fix the failing ACP test.
4. Observe that /goal and /subgoal are missing from the advertised ACP command list and are not handled with the existing standing-goal behavior.

Expected Behavior

ACP should provide parity with the existing gateway/CLI goal workflow:

• ACP advertises /goal and /subgoal alongside existing local commands such as /steer and /queue.
• /goal uses the existing hermes_cli.goals.GoalManager; it does not introduce a new goal engine.
• /subgoal mutates the same persisted goal state used by the rest of Hermes.
• After a normal ACP agent turn, active goals are evaluated with GoalManager.evaluate_after_turn().
• If the goal should continue, ACP queues the synthetic continuation prompt through the existing queued_prompts / session flow.
• If the goal is done, paused, or cleared, ACP does not queue another continuation.

Proposed Scope

Keep this narrow to ACP parity for existing behavior:

• Add goal and subgoal to ACP advertised slash commands.
• Add ACP slash handlers for:
  • /goal
  • /goal status
  • /goal <text>
  • /goal pause
  • /goal resume
  • /goal clear
  • /subgoal
  • /subgoal <text>
  • /subgoal remove <n>
  • /subgoal clear
• Reuse GoalManager for all goal state and decisions.
• Reuse ACP's existing queued prompt draining path for kickoff and continuation prompts.
• Preserve existing /steer, /queue, /help, /model, and other ACP command behavior.

Non-goals

• No redesign of the goal system.
• No new ACP-specific goal engine.
• No broad refactor shared between gateway, CLI, and ACP.
• No change to gateway or CLI goal semantics except where unavoidable for compatibility.

Acceptance Criteria

• available_commands_update includes goal and subgoal.
• /goal status reports empty state when no goal is active.
• /goal <text> creates an active standing goal and queues kickoff work through ACP session state.
• /goal pause pauses the goal and removes only synthetic goal continuations from state.queued_prompts.
• /goal resume resumes the existing goal without changing unrelated queued prompts.
• /goal clear clears the goal and preserves normal user /queue prompts.
• /subgoal list/add/remove/clear works against the active GoalManager state.
• Completed normal ACP turns call GoalManager.evaluate_after_turn().
• Continuing goals append at most one synthetic continuation prompt to state.queued_prompts.
• Done, paused, cleared, interrupted, or empty turns do not create continuation loops.
• Existing ACP /steer and /queue tests continue to pass.

Suggested Tests

• Extend ACP command advertisement tests to include goal and subgoal.
• Add ACP slash-command tests for /goal status/set/pause/resume/clear.
• Add ACP slash-command tests for /subgoal list/add/remove/clear and invalid input.
• Add post-turn continuation tests that verify continuation prompts use the existing queued_prompts / session flow.
• Run the focused ACP suites:

scripts/run_tests.sh tests/acp/test_server.py tests/acp_adapter/test_acp_commands.py -q