feat(cli): expose gateway serve command

[anhermon]

Closes ANGA-353

Thinking Path

1. ANGA-353 requires a minimal operator-facing WebSocket gateway path for Anvil.
2. The existing harness-gateway crate already owns the Axum WebSocket server, /health, ping/pong, event broadcast, and control-command plumbing.
3. The missing piece is a CLI surface that lets operators start that gateway without writing Rust or needing an LLM API key.
4. The CLI crate is the right integration point because it already exposes operator commands such as run, eval, memory, and paperclip.
5. Keeping the change in crates/cli avoids touching provider traits, Paperclip API adapter behavior, or gateway wire semantics.
6. anvil gateway serve should print the actual health and WebSocket URLs so users can verify the selected port, including OS-assigned port 0.
7. Startup event emission is opt-in via --emit-hello, preserving the default behavior as a passive local gateway.
8. --emit-hello should wait until a WebSocket client is subscribed, otherwise the broadcast channel correctly drops the smoke event before any operator can observe it.
9. CLI validation should reject invalid event-buffer values before server startup, so --event-buffer 0 cannot panic inside the broadcast channel.
10. Control commands sent through WebSocket must be drained while the standalone CLI server runs so the bounded command channel cannot fill and stall socket tasks.

What Changed

• Added a gateway top-level CLI command and gateway serve subcommand.
• Wired harness-gateway into harness-cli and starts Gateway::new(config).start() from the CLI.
• Added --port, --event-buffer, and --emit-hello options for local gateway startup.
• Prints /health and /ws URLs after successful bind, including the actual bound address.
• Drains and logs forwarded control commands while the CLI gateway is running.
• Defers --emit-hello startup events until the first WebSocket client has subscribed.
• Added CLI surface tests for the new command and for rejecting --event-buffer 0.
• Added gateway regression tests for subscriber counting and late-subscriber hello delivery.

Verification

• cargo fmt --check passes
• cargo test --workspace passes
• cargo clippy --workspace -- -D warnings clean
• cargo test -p harness-cli --test cli_surface passes
• cargo test -p harness-gateway passes
• Manual smoke: anvil gateway serve --port 19157; GET /health returned {"status":"ok"} and WebSocket {"cmd":"ping"} returned {"kind":"pong"}
• Pre-push hook passed (cargo test --workspace)

Risks

Low risk. The change only exposes an existing gateway crate through the CLI, adds argument validation, drains existing control-command traffic, and makes opt-in hello emission wait for a subscribed WebSocket client. The new server command binds to localhost through the existing gateway implementation and does not change provider traits, Paperclip adapter behavior, or gateway wire semantics.

Checklist

• Tests pass locally
• No new warnings from clippy
• Code is formatted with cargo fmt
• Commit messages follow conventional format
• Co-Authored-By trailer included for agent commits

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