Rewrite config to split state and hints architecture

[jeevanpillay]

Summary

Replace single dual.toml with a two-part architecture:

~/.dual/workspaces.toml — Centralized workspace state

• WorkspaceState struct with workspace_root + Vec of workspace entries
• Each entry: repo name, git URL, branch, optional explicit path
• New src/state.rs module with load/save/add/remove/resolve operations
• State persisted to disk, survives restarts

.dual.toml per repo — Runtime hints

• RepoHints struct: image (default: node:20), ports, setup command, env vars
• Lives in workspace directory root, checked into repo
• Read-only at runtime, written by dual add

CLI changes

• dual add — register current repo as workspace + create .dual.toml
• dual create <repo> <branch> — add branch workspace to state
• dual launch <workspace> — launch workspace
• dual list — list all workspaces
• dual destroy <workspace> — destroy workspace
• dual open, dual urls, dual proxy, dual shell-rc — retained

All consumers updated

• clone, container, proxy modules updated to use new state + hints APIs
• All 98 tests pass (64 lib + 14 main + 3 e2e + 7 fixture + 10 harness)

PR

• PR #112

Phase 1 issues advanced by this work

• DUAL-21: Workspace state machine — state persistence layer now implemented
• DUAL-22: Project service/port configuration — .dual.toml hints now cover ports, image, setup, env

[linear]

DUAL-42