feat: ACP-aware doctor checks

[jecanore]

Problem

researchclaw doctor always runs three HTTP-based checks (llm_connectivity, api_key_valid, model_chain) regardless of provider. ACP users see confusing failures for checks that do not apply — ACP uses a local agent, not an HTTP API.

Solution

• Add check_acp_agent() — uses shutil.which() to verify the agent CLI is on PATH
• When provider is "acp", run the agent check instead of the 3 HTTP checks
• Non-ACP configs still get all 9 checks; ACP configs get 7

Test plan

• check_acp_agent passes when agent found, fails when missing
• ACP config skips HTTP checks, includes agent check (7 total)
• Non-ACP config unchanged (9 checks)
• Full suite passes (1255 tests, 4 new)