docs(skills): add project-level upstream-rotation skill

[Gaubee]

Adds .agents/skills/upstream-rotation/ — the OpenSpec CLI upgrade-following workflow distilled from the 1.13.2 (13.0.2) and 1.14.2 (14.0.2) rotations.

• SKILL.md (156 lines): six gated phases P0-P6 (detect/triage → env → evidence report → change + dual-channel remix review → slice implementation + vision walkthrough → gates/PR → release/cleanup), plus a 10-row iron-rules table where every rule carries the incident that paid for it.
• references/ (4 files, on-demand): investigation (triage commands, mirror-surface table, report format), review-remix (herdr startup, Codex provider-sandbox probing incl. the inline-material mode, dual-channel convergence, round protocol), implementation (slice topology, subagent brief template, vitest v4 gate traps, whole-repo version-literal audit list, ego-browser walkthrough recipe), delivery (gates, anti-fake-merge, CI flake triage, four-fold release evidence, cleanup audit, visual-acceptance waiver criteria).

Quality: independent subagent review scored 9/10 — one blocker (a lexicographic ls | tail -1 silently pointing at the v9-era report template — ironically violating the skill's own rule 10) plus seven non-blocking suggestions, all folded before this commit. Docs-only; no code paths touched.