Migration path from `.dual.toml` to `.dual/settings.json`

[jeevanpillay]

Summary

Provide a smooth upgrade path for users with existing .dual.toml files. Detect the old format and either auto-migrate or guide the user.

Behavior

When load_hints() or dual init detects .dual.toml exists but .dual/settings.json doesn't:

1. Log a warning: "Found .dual.toml (deprecated). Run dual init to migrate to .dual/settings.json"
2. Fallback: still load from .dual.toml if .dual/settings.json is missing (graceful degradation)
3. dual init in a repo with .dual.toml:
   • Read existing values (extra_commands, anonymous_volumes, shared)
   • Pre-populate the init wizard with these values
   • Write .dual/settings.json
   • Prompt: "Remove old .dual.toml? [Y/n]"

Implementation

1. load_hints() fallback — if .dual/settings.json missing but .dual.toml exists, load from TOML with deprecation warning
2. cmd_init() migration — detect .dual.toml, read its values, use as defaults for new config
3. Keep TOML parsing temporarily — toml crate stays as dependency until migration period ends

Tests

• .dual.toml only → loads with warning
• .dual/settings.json present → .dual.toml ignored
• Migration populates settings.json from .dual.toml values

[linear]

DUAL-67