fix: isolate MCP stdout so tool-handler writes don't kill rmcp transport

[Desperado]

Problem

Codex orch sessions died mid tool-loop with:

ERROR rmcp::transport::worker: worker quit with fatal: Transport channel closed,
when Deserialize(Error("data did not match any variant of untagged enum JsonRpcMessage", line: 0, column: 0))
✗ codex exited with error: exit status 1

line: 0, column: 0 means rmcp tried to deserialize empty input — an empty line on the MCP server's stdout.

Root cause

The qmax MCP server (qmax-code serve --mcp) shared os.Stdout between the JSON-RPC transport and terminal UI code. Tool handlers — and the TUI helpers they call — freely fmt.Print to os.Stdout:

• runTestWithProgress calls fmt.Println() (writes \n → empty line) at internal/agent/tools.go:1043
• internal/tui/* writes progress bars, browser animations, banners via fmt.Print*

These non-JSON bytes landed on the same newline-delimited stream Codex's rmcp client reads. An empty line from fmt.Println() deserialized to nothing (serde line 0, column 0); a progress bar deserialized to garbage. Either kills the transport worker → Codex exits 1.

Fix (internal/mcp/server.go)

1. Stdout isolation — RunServer pins the real stdout for exclusive use by the JSON-RPC encoder, then repoints the process-level os.Stdout at stderr. Every stray fmt.Print from a tool handler now lands on stderr (diagnostics), never on the wire the parser reads.

2. Panic recovery — dispatch gets a deferred recover so a panicking tool handler (nil deref, index out of range) yields a -32603 JSON-RPC error response instead of crashing the server. A crash would EOF stdout and kill the transport the same way.

3. Extracted serveMCP(in, out) — the read/respond loop is now testable without swapping global os.Stdin/os.Stdout.

Tests (internal/mcp/server_test.go)

• TestDispatchRecoversFromPanic — nil-deref in agent.ExecuteTool is caught by the deferred recover and returns a -32603 response.
• TestServeMCPOutputIsCleanJSON — initialize → notifications/initialized → tools/list handshake produces only valid JSON-RPC lines (no empty or corrupt lines), and the notification is not echoed.

Test plan

• go test ./internal/mcp/ -v — all 5 tests pass
• go test ./... — full suite green
• go vet / gofmt clean
• Manual: run a Codex orch session that triggers qmax tools (e.g. run_test) and confirm the tool loop completes without the rmcp transport fatal

[qualitymaxapp]

✅ QualityMax Pipeline

Gate	Result
🔍 AI Review	✅ Clean
🧪 Repo Tests	✅ 406/406 passed (go)
🤖 AI Tests	✅ 28/32 passed

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