/investigate skill logs type:"investigation" but gstack-learnings-log silently rejects it

[diogolealassis]

gstack v1.31.1.0

investigate/SKILL.md:959 tells the agent to log learnings with type:"investigation",
but bin/gstack-learnings-log:22 has:

const ALLOWED_TYPES = ['pattern','pitfall','preference','architecture','tool','operational'];

So every /investigate run silently drops its learning (exit 0, no write).
Users lose all investigation history without warning.

Fix: add "investigation" to ALLOWED_TYPES, OR change the SKILL.md template
to use one of the existing types (e.g. "pitfall" with source:"observed").

[jbetala7]

Opened PR #1426 to fix this: #1426