fix(mcp): return structured compact results

[WarGloom]

What does this PR do?

Every MCP tool advertises an object outputSchema, but compact TOON and
plain-text results omitted structuredContent. Strict MCP clients therefore
rejected otherwise successful tool calls with -32600.

This change keeps JSON-object results unchanged and adds an object for every
other result:

• successful text and TOON payloads use {"text":"..."}
• error text uses {"error":"..."}

The existing text content block remains for model visibility and backwards
compatibility.

Validation

• ASAN_OPTIONS=detect_leaks=0 make -f Makefile.cbm test — 6,009 passed,
  1 skipped
• make -f Makefile.cbm lint-ci
• wire-level coverage for compact search_graph responses plus unit coverage
  for plain-text and error results

Checklist

• Every commit is signed off (git commit -s) — required, CI rejects
  unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (make -f Makefile.cbm test)
• Lint passes (make -f Makefile.cbm lint-ci)
• New behavior is covered by a test (reproduce-first for bug fixes)