refactor(docs): rename STEEV-MASTER.md → CONTRACT.md (SOT taxonomy)

Per the workspace SOT classification: Tier S contracts use the canonical filename CONTRACT.md at repo root. STEEV-MASTER.md → steev/CONTRACT.md.

- File moved from docs/STEEV-MASTER.md to CONTRACT.md (repo root)
- Frontmatter note updated
- CLAUDE.md structure tree updated
- Empty docs/ directory removed

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -27,7 +27,7 @@ steev/
 │   ├── steev-agent/                 # orchestrator skill (SKILL.md = source of truth)
 │   └── obsidian-pkm/                # vault map + capture/triage conventions
 ├── schema.sql                       # steev.db schema
-└── docs/                            # STEEV-MASTER.md
+└── CONTRACT.md                      # Steev profile contract — tier S (this file wins)
 ```
 
 ## Gotchas

CONTRACT.md

@@ -4,7 +4,7 @@ status: active
 owner: jp
 source: hand
 review_by: 2026-08-21
-note: "Steev profile contract; future rename: steev/CONTRACT.md"
+note: Steev profile contract — this file wins for the Steev profile
 ---
 
 # Steev — Source of Truth