Accept implementation extensions as spec updates based on UAT findings:
1. `/session:export` now officially supports three formats:
- `json` (default, canonical, importable)
- `md` (Markdown transcript, lossy)
- `txt` (plain-text transcript, lossy)
Updated ADR-046 command table and spec Conversation Export section.
2. Persona data model now includes two optional display fields:
- `icon`: emoji or single character for tab labels and lists
- `greeting`: message shown when persona is activated
Updated ADR-045 data model YAML example and Decision section.
These changes accept positive implementation extensions discovered by
UAT testing (issues #4871, #4858) rather than reverting them.
Refs: #4940
freemo