Re-enable seambot.yml against gitbot-fleet (pinned ref) #39

Sub-issue of #37.

seambot.yml clones the phantom hyperpolymath/seambot. seambot actually lives in hyperpolymath/gitbot-fleet at bots/seambot and is a genuine repo-scanning CLI (seambot check, binary seambot).

Fix: clone gitbot-fleet at a pinned SHA, cargo build --release in bots/seambot, run seambot check from the gitbot-fleet bots/seambot/target/release/seambot, keep the existing spec/seams/seam-register.json guard, drop if: false. No upstream change needed.

Refs #37.