feat(tui): add /instructions command to show loaded instruction files

[phall1]

What does this PR do?

Closes #7679

Add a slash command that displays which instruction files (AGENTS.md, CLAUDE.md, config-specified files, URLs) are loaded for the current session.

Changes:

• Extract path discovery from SystemPrompt.custom() into reusable paths() function
• Add /instructions server endpoint via separate route file (avoids TS2589)
• Add DialogInstructions TUI component
• Register command in autocomplete and command palette

How did you verify your code works?

• test & bun run dev