Add agent constraints — per-session and global guardrails (Settings UI + /constraints slash command)

[neerajdad123-byte]

Add agent constraints — per-session and global guardrails (Settings UI + /constraints slash command)

Description:
This PR adds a new Constraints feature that lets users set permanent guardrails the agent must follow — things like
"never edit .env files" or "only work in src/."

What it does:

• New Constraints section in Settings with scope toggle (Session / Global), text input, Add and Remove buttons
• /constraints slash command opens settings directly to the Constraints section
• Constraints are prepended to every prompt as a XML block the agent sees
• Session constraints persist per-project in workspace state; global constraints persist across all projects in
  localStorage
• Fully backward-compatible — old workspace data without constraints defaults to empty

How to test:

1. Open the app → Settings → Constraints (or type /constraints)
2. Select "Global" scope, type "never edit .env files", click Add
3. Switch to "Session" scope, add "only suggest changes, don't apply them"
4. Go to chat, type a prompt — the agent receives both constraints
5. Remove a constraint via the ✕ button
6. Restart the app — global constraints persist, session constraints persist per project

Files changed: 6 files, +178 −4

• New: src/rpc/constraints.ts — shared module for constraint storage and prompt prefix generation
• src/main.ts — workspace state field, persistence wiring, /constraints routing
• src/components/settings-panel.ts — Constraints section UI and state
• src/components/chat-view/send-message-flow.ts — prepend constraints to prompts
• src/commands/slash-command-catalog.ts — /constraints registration
• src/components/chat-view/slash-builtin-command.ts — /constraints handler

Screenshot: