Refactor workspace_init.sh into a `shock init` subcommand

[jerrod-storm]

Problem

Shellshock's onboarding for new consumer repos is clunky. workspace_init.sh exists but it's a standalone script that doesn't integrate with the shock CLI, and it doesn't handle .envrc setup.

Proposal

Replace workspace_init.sh with a shock init subcommand that:

1. Does everything workspace_init.sh currently handles (submodule setup, directory structure, etc.)
2. Creates or patches the consumer repo's .envrc to source Shellshock's .envrc and export PROJ
3. Provides a single, discoverable entry point for setting up Shellshock in a new or existing repo

Motivation

With the new .envrc-based environment bootstrapping (where consumer repos source $PROJ/shell/.shock/.envrc), init needs to wire that up automatically. Having it as a subcommand means it follows the same patterns as other Shellshock operations.

Note: BASH_SOURCE dirname pattern

The BASH_SOURCE dirname idiom (DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)") is repeated across test helpers and .envrc files. A shared utility for this was considered but has a chicken-and-egg problem: you need to know where Shellshock is to source the utility that resolves paths. shock init is a better place to address this -- it can generate the correct boilerplate with the right paths baked in, rather than trying to abstract the idiom into a library function.

Acceptance Criteria

• shock init performs all current workspace_init.sh functionality
• shock init creates/edits .envrc with correct PROJ export and Shellshock .envrc source
• workspace_init.sh is removed or deprecated
• Running shock init on an already-initialized repo is safe (idempotent)