docs(en): add bmad-investigate / IN trigger to agent tables (#2410)

The forensic investigation feature (commit 380590aa) added the IN menu
trigger and bmad-investigate skill, but several docs that enumerate
triggers and agent capabilities were not updated.

- agents.md: add IN trigger and Forensic Investigation to Amelia's row
- named-agents.md: add forensic investigation to Amelia's capabilities

Co-authored-by: Brian <bmadcode@gmail.com>