Rename `dual add` → `dual init`

[jeevanpillay]

Summary

Rename the dual add command to dual init. This better reflects what the command does — it initializes a repo as a Dual workspace, creating config files and registering it in global state.

Breaking Change

dual add → dual init. The add subcommand is removed.

Implementation

1. src/cli.rs — Rename Add variant to Init, update clap attributes
2. src/main.rs — Rename cmd_add() → cmd_init(), update match arm
3. User-facing messages — Update all references from "dual add" to "dual init" across info/error messages
4. README.md — Update CLI command table and quick start section

Tests

• CLI parsing tests updated for dual init
• Existing add_subcommand and add_with_name tests renamed

[linear]

DUAL-62