fix(app): invoke skills anywhere in prompts - #52055

Issue for this PR

Closes #52051

Related to #40689.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Desktop/Web slash autocomplete only recognized commands at the beginning of the prompt. Selecting a Skill after entering other text could also replace the draft or submit the Skill name as ordinary prompt text.

This change detects the slash query immediately before the cursor anywhere in the prompt, including multiline drafts. Selecting a server-backed command replaces only that query, preserves the surrounding text and attachments, and renders the selected command as a blue token. Submitting the draft executes that command and passes the surrounding text as its arguments.

Built-in client commands keep their existing behavior.

How did you verify your code works?

• Tested completion at the beginning, middle, and end of a prompt and on a later line.
• Verified selection preserves text before and after the slash query.
• Verified the selected command renders as an RGB (69, 130, 204) token.
• Verified submission sends a command request with the surrounding draft as arguments.
• Ran 92 Session UI prompt tests and 22 prompt state/history/request-building tests.
• Ran 8 Playwright regressions covering both the legacy and V2 prompt inputs.
• Manually verified the flow in OpenCode Desktop.

Screenshots / recordings

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR