UX: move context hints into input placeholder as a single line

Currently the context attachment hints appear as a separate line above the input box:

@cwd @tab  • type @ to attach
┌─────────────────────────────┐
│                             │
└─────────────────────────────┘

Suggestion: collapse everything into a single placeholder sentence inside the input:

┌──────────────────────────────────────────────┐
│  Type @ to attach context or / for commands  │
└──────────────────────────────────────────────┘

The placeholder disappears once the user starts typing. This removes the permanent extra line above the input and keeps the same guidance with less visual noise.