feat(ui): surface --dir / pinned workspace for first-time users

[esengine]

Closes #370.

Why

Beginners hit the default cwd-scoped behavior and conclude Reasonix can't read folders other than where they launched it. reasonix code --dir <path> exists today and works fine — it's just invisible until you --help. (The issue body suggested /cwd exists; it doesn't, and the agent's system prompt explicitly says mid-session retargeting was removed because the message log + memory paths get tangled with a stale root.)

So this is purely an onboarding fix — no new capability, no /cwd slash.

Surfaces added

1. WelcomeBanner — shows a workspace line in code mode on the empty state:

   ▸ workspace · /path/to/repo  (relaunch with --dir <path> to switch)
   

2. /status — adds a workspace line:

   workspace /path/to/repo · pinned at launch (relaunch with --dir <path> to switch)
   

3. Filesystem sandbox-escape error — was a raw path escapes sandbox root (X): Y; now points the user at the right fix:

   path escapes sandbox root (X): Y — workspace is pinned at launch; quit and relaunch with reasonix code --dir <path> to work in a different folder

4. README + README.zh-CN — Getting Started subsection explaining --dir and why mid-session switching isn't a thing.

Tests

No new test file — all four surfaces are static text additions. Existing tests cover the unchanged code paths:

• tests/filesystem-tools.test.ts — still asserts /escapes sandbox/ (matches the new message prefix).
• tests/slash.test.ts — unaffected (workspace line only appears in /status when ctx.codeRoot is set).

Test plan

• npm run verify — 134 files / 2152 passed / 1 skipped
• Manual: reasonix code empty-state shows the workspace banner line
• Manual: /status shows the workspace line in code mode
• Manual: ask the model to read a path outside rootDir → tool error mentions reasonix code --dir