feat(gateway): command hooks for deterministic chat commands

[bkuri]

Problem

Every chat message today — including slash commands — costs a full backend turn. Gateway built-ins (/clear, /help, /stop) short-circuit, but anything user-defined has to route through the LLM even when the desired behavior is fully deterministic (e.g. /status running a status script and relaying its output).

Change

Adds a [command_hooks] config table mapping chat slash commands to shell commands, handled in the existing gateway command path before backend dispatch:

• Reply is the hook's trimmed stdout, relayed verbatim; no agent turn.
• Message arguments are appended to the command line as trailing positional args (/status agents → status-report agents).
• Failure, empty output, and timeout (>15s) reply with a short deterministic error — a broken hook never silently becomes an LLM turn.
• Unknown slash commands still reach the backend as before, so backend-side command fallbacks keep working.
• Hooks run through /bin/sh (POSIX), stdout+stderr captured, kill_on_drop on timeout.

Verification

• cargo fmt --all --check, cargo clippy --locked --all-targets -- -D warnings, cargo build --locked, cargo test --locked all pass (448 tests).
• New tests: hook stdout relay + arg passthrough without any backend call; unknown-command fallthrough to the backend; docs build (mkdocs build --strict).

Risk

Low: default config (command_hooks empty) is behavior-identical to today. The command() signature gains async — call site is already async context. Docs updated on the canonical page (docs/reference/cli.md).

Note: the hardened hook runner in #197 (process groups, stdout cap) is a better execution engine for this path; this PR deliberately ships the simpler runner to stay focused, and the two can be unified when #197 lands.