Deprecate --tunnel flag and rc/rc-tunnel/start --tunnel commands #899

Summary

Deprecate and eventually remove the tunnel-related CLI commands and flags:

• squad start --tunnel
• squad rc (Remote Control bridge)
• squad rc-tunnel (devtunnel check)

Phase 1 (this issue): Deprecation warnings

• Add console warnings when these commands/flags are used
• Update --help output to mark these as deprecated
• No behavior changes — everything still works, just warns

Phase 2 (future): Removal

• Remove the commands entirely
• Clean up any related code paths

Why

The tunnel/RC features add complexity to the CLI without sufficient adoption. The Copilot CLI provides native remote access capabilities. Squad should be lean — a command-line toolkit, not an interactive server.

Related

• Remove the REPL experience — CLI should be commands only, no interactive session #665 — Remove the REPL experience (same simplification direction)