feat: adopt symlink approach for cross-tool instruction files

Replace 4 separate instruction files with 1 real file + symlinks:
- .github/copilot-instructions.md is the canonical instruction file
- CLAUDE.md and .cursorrules are symlinks to it
- AGENTS.md stays as global config (not a project instruction file)

This eliminates all sync/merge logic - changes propagate automatically.

Changes:
- AGENTS.md: rewrite Cross-Tool Instruction Files section for symlink convention
- librarian.md: simplify to maintain single instruction file + verify symlinks
- lead.md: simplify PHASE-WRAP and Documentation Completion Gate
- commands (init, bootstrap-memory, save-memory): update for symlink model
- doc-coverage skill: verify symlinks exist and point correctly

.config/opencode/.github/copilot-instructions.md

@@ -18,10 +18,9 @@ agents/                     # Agent definitions (one .md per agent)
   sme.md                    # Domain expert (writes .memory/*)
   designer.md               # UI/UX guidance (writes .memory/*)
 .memory/                    # Persistent project memory (knowledge, decisions, plans, research)
-AGENTS.md                   # OpenCode instruction file
-CLAUDE.md                   # Claude Code instruction file
-.github/copilot-instructions.md  # This file (GitHub Copilot instruction file)
-.cursorrules                # Cursor instruction file
+AGENTS.md                   # Global OpenCode config (not a symlink in this repo)
+CLAUDE.md -> .github/copilot-instructions.md
+.cursorrules -> .github/copilot-instructions.md
 ```
 
 ## Agent Permission Model
@@ -47,6 +46,6 @@ Three tiers based on change scope (Tier 1: full, Tier 2: standard, Tier 3: fast)
 
 ## Conventions
 
-- Cross-tool instruction files (`AGENTS.md`, `CLAUDE.md`, `.github/copilot-instructions.md`, `.cursorrules`) must stay synchronized.
-- `.memory/knowledge.md` is the canonical source of truth; instruction files are derived views.
+- This repo uses symlinks for cross-tool instruction files: `CLAUDE.md` and `.cursorrules` point to this file.
+- `.memory/knowledge.md` is the canonical source of truth for deep project knowledge.
 - Recording discipline: record only outcomes/decisions/discoveries in `.memory/`, never ceremony checkpoints.