Replace bash shim with Rust binary

Bash shim had shell quoting risks and depended on socat/nc. Elixir
escript would pay ~300ms BEAM boot per invocation. A second Burrito
binary would unpack on every cold call. Rust gives <1ms startup,
proper timeout handling, and is already in the toolchain for the
tree-sitter NIF.

- Add shim/ Rust crate to directory structure
- Document Rust shim rationale (vs bash, escript, Burrito)
- Update Dependencies with shim crate deps (serde_json, stdlib Unix socket)
- Update install script, README, architecture diagram

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/superpowers/specs/2026-03-26-security-hooks-design.md

@@ -13,16 +13,19 @@ A general-purpose, distributable set of security hooks for AI coding agents (Cla
 
 Four components:
 
-### 1. Shell shim (`security-hook`)
+### 1. Shim (`security-hook`)
 
-A short bash script that AI coding tools invoke as a hook command. It:
+A small Rust binary (~1MB static, <1ms startup) that AI coding tools invoke as a hook command. Rust is chosen over bash to avoid shell quoting bugs, over Elixir escript to avoid ~300ms BEAM boot, and over a second Burrito binary to avoid unpack-on-every-invocation overhead. Since tree-sitter-bash already requires Rust in the build toolchain, this adds no new dependencies.
+
+The shim:
 
 - Accepts an `--adapter` flag to specify the calling tool (`claude`, `gemini`, `codex`)
 - Reads the JSON hook payload from stdin
-- Passes the payload and adapter name to the daemon over a Unix socket
-- Prints the daemon's response to stdout (formatted for the calling tool)
+- Connects to the daemon's Unix socket and sends the payload with the adapter name
+- Reads the daemon's response and prints it to stdout
+- Handles timeouts and fail-closed behavior natively (no shell `timeout` command)
 
-The shim is deliberately simple — it does not manage daemon lifecycle. That responsibility belongs to the platform service manager (see Daemon Lifecycle below).
+It does not manage daemon lifecycle. That responsibility belongs to the platform service manager (see Daemon Lifecycle below).
 
 Usage:
 ```
@@ -147,8 +150,10 @@ When installed, the entire tree below is copied to `$SECURITY_HOOKS_HOME` (defau
 
 ```
 security-hooks/
-├── bin/
-│   └── security-hook                    # shell shim
+├── shim/                                # Rust shim binary
+│   ├── Cargo.toml
+│   └── src/
+│       └── main.rs
 ├── service/
 │   ├── security-hookd.service           # systemd user service unit
 │   ├── security-hookd.socket            # systemd socket activation unit
@@ -838,7 +843,7 @@ Future: streaming connectors for centralized logging (stdout, webhook, syslog).
 
 The install script:
 1. Downloads the Burrito binary for the current platform (macOS aarch64/x86_64, Linux x86_64/aarch64) or builds from source if Elixir is available
-2. Installs the binary and shell shim to `~/.local/bin/` (or user-specified location)
+2. Installs both binaries (`security-hookd` daemon + `security-hook` shim) to `~/.local/bin/` (or user-specified location)
 3. Copies default rules and config to `~/.config/security-hooks/`
 4. Creates `config.local.toml` from a template if it does not exist
 5. Auto-detects installed MCP servers from Claude Code/Gemini CLI config and pre-populates the MCP allowlist in `config.local.toml`
@@ -981,5 +986,10 @@ Elixir/Hex packages required by the daemon:
 - `file_system` — cross-platform file watcher for hot-reload
 - `burrito` — compile to single-binary for distribution
 
-Rust NIF (compiled into the Burrito binary):
+Rust (compiled into the Burrito binary as a NIF, and used standalone for the shim):
 - `tree-sitter` + `tree-sitter-bash` — bash AST parser for structural command analysis (primary parser, see Bash Parser Strategy)
+
+Shim binary (`security-hook`, Rust):
+- `std::os::unix::net::UnixStream` — Unix socket client (stdlib, no external deps)
+- `serde_json` — JSON parsing
+- Cross-compiled for the same platform targets as the daemon