feat: distinguish empty results from broken index in search output

[ktappdev]

Problem

contexting search-hints "query" --json returns [] in two very different situations:

1. No matches — index exists and is healthy, query just didn't match anything
2. Broken/missing index — index file doesn't exist or is corrupted, search silently returns nothing

LLM agents and tools can't tell the difference. They assume "empty results = nothing relevant" when it might mean "your index is broken."

Proposed Solution

Wrap JSON output in an envelope with metadata:

{
  "source": "memory | snapshot | none",
  "index_generated_at": "2026-06-16T10:00:00Z",
  "results": [
    {"path": "src/auth.go", "type": "file", "score": 85, "matches": ["..."]}
  ]
}

Where source:

• "memory" — results from live watch server
• "snapshot" — results from .ctx/ctx_index.json file
• "none" — no index available, results always empty

Implementation Notes

• Modify resultsToJSON or create new envelopeToJSON wrapper
• Track source in the search flow (already have usedMemory bool in command_search.go:84)
• index_generated_at from ContextIndex.GeneratedAt (already in index JSON)
• For --dir-summary mode, add same envelope
• Non-JSON output (--json=false) unchanged

Acceptance Criteria

• --json output includes source field
• source: "none" when index missing or corrupted
• source: "memory" when connected to live watch server
• source: "snapshot" when reading from index file
• index_generated_at included when available
• Backward compatible: results array has same shape as current output