Remove mesh module — replaced by server architecture

[marklubin]

The synix mesh module (distributed builds via Raft-like consensus) is dead code now that we've moved to the server architecture (synix serve).

Remove:

• src/synix/mesh/ (entire package — server, client, dashboard, health, auth, logging, etc.)
• tests/mesh/
• CLI commands under synix mesh
• [project.optional-dependencies] mesh in pyproject.toml
• Any mesh-related docs

The mesh dashboard's activity panel (log tailing + display) might be worth salvaging into the web viewer as a future enhancement, but the mesh infrastructure itself should go.