fix: remove max-w-6xl from instructions tab (#2621)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies
> - The web UI includes an agent detail page with an Instructions tab
for editing agent prompts
> - The Instructions tab used `max-w-6xl` (1152px) to constrain its
two-panel layout (file tree + editor)
> - The floating Cancel/Save buttons used `float-right` at the full page
width, disconnecting them from the constrained content
> - This also left a large empty margin on the right side at wider
viewports
> - This pull request removes `max-w-6xl` so the flex layout fills
available width
> - The benefit is buttons now align with the content edge, and the
right margin is eliminated

## What Changed

- Removed `max-w-6xl` from the `PromptsTab` container in
`ui/src/pages/AgentDetail.tsx:1920`
- The file panel + editor flex layout now fills the available page width
naturally

## Verification

- Navigate to any agent's Instructions tab at a wide viewport (1920px+)
- Before: content stops at 1152px with a gap to the right; Cancel/Save
buttons float to the far edge
- After: content fills available width; Cancel/Save buttons sit flush
with the editor panel

## Risks

- Low risk — only removes a max-width constraint on a single tab's
container. Other tabs (Configuration, Skills, etc.) are unaffected.

## CI Note

The test failure in `feedback-service.test.ts:714` is **pre-existing**
and unrelated to this change. A PII redaction filter is incorrectly
treating a UUID segment (`5618-4783`) as a phone number, producing
`[REDACTED_PHONE]` in the expected UUID value.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [ ] I have run tests locally and they pass
- [ ] I have added or updated tests where applicable
- [x] If this change affects the UI, I have included before/after
screenshots
- [ ] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

ui/src/pages/AgentDetail.tsx

@@ -1917,7 +1917,7 @@ function PromptsTab({
   }
 
   return (
-    <div className="max-w-6xl space-y-6">
+    <div className="space-y-6">
       {(bundle?.warnings ?? []).length > 0 && (
         <div className="space-y-2">
           {(bundle?.warnings ?? []).map((warning) => (