Add a /credits command so CLI users can check their balance and buy more

[nightnei]

Fixes STU-2327

How AI was used in this PR

Assisted

Proposed Changes

With this PR we are adding /credits command to see info about remaining balance and top-up options

Testing Instructions

1. npm run cli:build && node apps/cli/dist/cli/main.mjs code
2. Type /credits. Expect your balance, then a picker listing every priced top-up:
   
3. Press Enter on one — checkout opens at that quantity.
4. Press Esc — /credits are closed
5. /logout, then /credits — expect the /login prompt.
6. Exhaust credits and send a prompt:
   

[wpmobilebot]

📊 Performance Test Results

Comparing e7fe40b vs trunk

app-size

Metric	trunk	e7fe40b	Diff	Change
App Size (Mac)	1422.09 MB	1422.08 MB	0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	e7fe40b	Diff	Change
load	1189 ms	1198 ms	+9 ms	⚪ 0.0%

site-startup

Metric	trunk	e7fe40b	Diff	Change
siteCreation	7530 ms	7519 ms	11 ms	⚪ 0.0%
siteStartup	3367 ms	3367 ms	0 ms	⚪ 0.0%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)

[gcsecsey]

Thanks @nightnei for tackling this! Changes LGTM and work well.

State	Output
/credits
Hitting Enter
Hitting Esc
Using /credits while logged out
Having run out of credits

[Poliuk]

@nightnei in the 6 - Exhaust credits and send a prompt: let's remove the "or use the link below: http://wordpress.com/checkout/wpcom/studio-code-ai-credits:-q-100000" part, as that will limit visibility of the other top-up options.

cc/ @gcsecsey