Route logs to stderr in MCP mode

[MadTinker]

Stacked on #4. Base is claude/project-roadmap-review-qlb7j0, so the diff shows only this fix.

The bug

MCP speaks JSON-RPC 2.0 over stdout. The tracing subscriber also defaulted to stdout, so every log line was interleaved into the protocol stream.

A client reading the first line for its initialize response got this:

INFO ThreadId(01) tinker: src/main.rs:90: Starting Tinker Workshop...

instead of JSON, and failed to parse. This affected any MCP client, including Claude Desktop using the configuration in the readme.

Reproduced by discarding stderr and observing that log lines still arrive on stdout:

$ tinker --mcp --url https://example.com < init.json 2>/dev/null
INFO ThreadId(01) tinker: src/main.rs:90: Starting Tinker Workshop...
INFO ThreadId(01) tinker::browser::console: Creating console monitor...
...

Two causes

1. tracing_subscriber::fmt() writes to stdout unless told otherwise.
2. Logging was initialized at line 80, before Args::parse() at line 93 — so it could not know whether --mcp was set. The info! on line 89 was the first line to corrupt the stream.

The fix

Parse arguments first, then point the subscriber at stderr when running as an MCP server. Non-MCP output is unchanged — same destination, same format.

The MCP write loop was already correct (writes JSON, flushes) and there are no other println!/print! calls anywhere in src/, so the subscriber was the sole corruption source.

The ignored tests were masking this

tests/mcp_tests.rs had three #[ignore]d tests whose comment read "Ignore by default since it requires building the binary." That wasn't why they were failing — they were failing on the corrupted stream, at the JSON parse on line 51.

They pass now and are re-enabled. They spawn the real binary and speak JSON-RPC end to end, so they guard this exact regression.

Verification

$ tinker --mcp --url https://example.com < init.json 2>/dev/null
{"jsonrpc":"2.0","id":1,"result":{"capabilities":{...},"protocolVersion":"2024-11-05","serverInfo":{"name":"tinker-browser-mcp","version":"0.1.0"}}}

Valid JSON, correct id, correct protocolVersion.

cargo test → 167 passed, 0 failed, 0 ignored

Previously 164 passed with 3 ignored.

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