test(mcp): agent smoke test across five coding agents, tool snapshot

- tests/agents/agent-smoke.sh runs tests/agents/cases.json through
  Claude Code, Codex, pi, Hermes Agent, and opencode headless against a
  local idx-mcp build, each with an isolated config, and checks the
  tools called, their key arguments, and (mock data) facts in the
  answer. Opt-in; never part of the gate.
- crates/idx-mcp/tests/golden/tools.json pins tool names, descriptions,
  and input schemas, so changes to what agents see are reviewed.
- nix develop .#agents adds Claude Code, Codex, and opencode from
  nixpkgs after the host PATH.

First run: 30/30 (5 agents x 6 cases, mock data).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

AGENTS.md

@@ -22,6 +22,7 @@ crates/idx-cli/             binary `idx`
 └── tests/                  cli.rs (behavior), golden.rs + golden/ (output parity)
 crates/idx-mcp/             binary `idx-mcp`: stdio MCP tools (server.rs), inputs (params.rs)
 └── tests/mcp.rs            JSON-RPC over stdio against the mock provider
+tests/agents/               opt-in agent smoke test: real coding agents against idx-mcp
 scripts/                    smoke, installer, and ownership snapshot publisher
 ```
 
@@ -52,4 +53,5 @@ Docs:
 nix flake check -L                       # canonical gate (fmt, clippy, tests, package)
 nix develop --command cargo test <name>  # focused loop
 scripts/live-smoke.sh --compare <old>    # live drift check, A/B against an older build
+tests/agents/agent-smoke.sh              # after changing idx-mcp tools: real agents, opt-in
 ```