fix(mesh): run relay-mesh agents on sprout-agent, not goose

[tlongwell-block]

What

Relay-mesh agents now run on the sprout-agent runtime instead of goose.

mesh_llm::agent_preset set agent_command = DEFAULT_AGENT_COMMAND (goose) and mcp_command = DEFAULT_MCP_COMMAND (sprout-mcp-server) while emitting SPROUT_AGENT_PROVIDER=openai + OPENAI_COMPAT_* env vars. Goose ignores those vars and falls back to its own configured provider, so mesh agents never used the local mesh endpoint.

The env set was already correct — it's exactly what crates/sprout-agent/src/config.rs reads. It was just being handed to the wrong runtime.

Change

• agent_command / mcp_command now sourced from the sprout-agent provider catalog entry (known_acp_provider_exact) → sprout-agent / sprout-dev-mcp, with consts as fallbacks. Can't drift from the provider definition.
• Env vars / base_url / model / api_key: unchanged (already correct for sprout-agent).
• Extracted the preset into a sibling preset.rs via the existing #[path] pattern — mod.rs was at 495 lines (the 500-line budget ceiling); this keeps it under and isolates preset logic. Reads as: mod.rs −48 (move-out), preset.rs +69 (move-in + the fix), mod_tests.rs +32 (regression test).
• Added regression test agent_preset_runs_on_sprout_agent_not_goose.

Matches the frontend applyMeshAgentPreset.test.mjs contract, which already expected agentCommand: sprout-agent / mcpCommand: sprout-dev-mcp.

Verification

• Full cargo test -p sprout-desktop --features mesh-llm --lib: 441 pass + new regression test
• No-feature gates (pre-push parity, run sequentially): desktop-tauri-clippy, desktop-tauri-test, workspace clippy, test-unit — all green
• Frontend preset tests 10/10
• fmt + clippy clean

Stacked on #846.