feat: context blocks — configurable per-message context injection

[dimakis]

Summary

• Config: Parse contextBlocks from .mitzo.json — maps names to file paths (relative or absolute)
• API: /api/config exposes available blocks with file sizes for the frontend picker
• Prompt assembly: Injects <context> XML blocks with a preamble ("The user has attached the following reference files…") and ---CONTEXT_END--- separator before the user's message
• Server wiring: contextBlocks flows through all WS → startChat/sendToChat/interruptChat → assemblePrompt paths
• Frontend: @ button in command strip opens a multi-select ContextPicker, selected blocks shown as removable pills above the textarea, cleared after send. User bubble shows compact @ Name1, Name2 annotation

Config example

{
  "contextBlocks": {
    "workflow": "context/workflow.md",
    "org": "/absolute/path/org_structure.md"
  }
}

Test plan

• 4 tests for config parsing (happy path, missing, non-string filter, absolute paths)
• Routes test updated for contextBlocks in mock config
• 8 tests for prompt assembly (plain, empty array, preamble injection, multiple blocks, unknown names, missing files, source path in tag, alongside images)
• Manual: add contextBlocks to a repo's .mitzo.json, verify picker shows entries with sizes
• Manual: select blocks, send message, verify preamble + XML in assembled prompt (check server logs)
• Manual: verify pills clear after send and user bubble shows @ name annotation

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