Update all consumers + user messages for `.dual/` model

[jeevanpillay]

Summary

Update all code that references .dual.toml to use .dual/settings.json. Update user-facing messages, error text, and info output.

Files to update

src/main.rs

• cmd_init() — creates .dual/ dir + settings.json (done in init issue)
• cmd_launch() — load_hints() already handles this
• cmd_sync() — error message references .dual.toml [shared] → update to .dual/settings.json
• All info/error messages mentioning .dual.toml

src/proxy.rs

• Empty ports message: reference devcontainer.json (already done, verify)

User-facing messages to update

Current	New
"no [shared] section in .dual.toml"	"no shared files in .dual/settings.json"
"Created .dual.toml"	"Created .dual/settings.json"
"Edit .dual.toml to customize..."	"Run dual init to configure..."
"Add 'forwardPorts' to devcontainer.json"	Keep as-is

Tests

• Grep for all .dual.toml string literals → verify none remain in non-migration code

[linear]

DUAL-66