Reject `:` character in schema field keys (breaks JSON-LD compaction)

**Issue found during schema testing:** two fields in a schema had a `:` character in the field key. Guardian accepted the schema on import/save, but the `:` is a reserved JSON-LD character (used for prefix:term compaction), and it produced a JSON-LD validation error downstream. Had to manually rename the two fields to unblock testing.

**Ask:**
1. **Validation** — Guardian should flag/reject `:` in a schema field key at authoring/import time (Excel and in-app schema editor), with a clear error message pointing to the offending field, rather than letting it pass silently and fail later at JSON-LD validation.
2. **Docs** — Add a note to schema authoring documentation about reserved/disallowed characters in field keys (starting with `:`).
3. **Excel template README tab** — Add the same reserved-character guidance to the README/Guidance tab generated with Excel schema templates, so authors see it at the point of authoring.

**Why now:** related to the existing "Lack of documented validation rules when uploading schemas via Excel" work (GRD-73, done) — this is a specific validation gap that slipped through that effort.