Add a merge-train skill for the main-session merge loop

[DocGerd]

Codify the hand-driven merge lifecycle into a skill (not an agent — the watcher agent is what oversleeps). Captures: self-armed check-runs Monitor → verify head.sha == pushed SHA → merge → update-branch re-sync of the next serial PR, plus the #94 504-reconcile and #119 stale-SHA rerun procedures.

Acceptance criteria

• Skill under .claude/skills/merge-train/ with SKILL.md.
• Encodes SHA-verify, serial re-sync, fix(routing): stamp visited only for frontier-cap survivors (#67) #94 504-reconcile, fix(pwa): dodge Pages CDN gzip-of-range on the basemap archive (#118) #119 stale-SHA rerun.
• Run through skill-creator quick_validate.py (per memory note).