feat: add shared workflow and skill references in .agents and update GEMINI.md for clarity on workspace structure

GEMINI.md

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+# Gemini Workspace Entry
+
+Use this file as the Gemini CLI entry point for this workspace.
+
+Shared rules and context already live in the normal workspace files. Do not duplicate or reinterpret them here.
+
+Read these first:
+
+@./AGENTS.md
+@./README.md
+@./agent-memory/README.md
+@./project-knowledge/00-start/start-here.md
+@./project-knowledge/01-current/current-work.md
+@./project-knowledge/01-current/work-items.md
+
+## Tool Surface
+
+- `AGENTS.md` is the primary shared rule file.
+- `.agents/` is the canonical shared cross-platform surface for workflows and reusable rule/skill content.
+- `.opencode/` is the OpenCode compatibility/runtime surface.
+- `.agent/` is an alias for tools that expect the singular directory naming.
+- `project-knowledge/` is canonical project memory.
+- `agent-memory/` is agent operating memory.
+
+## Gemini-Specific Notes
+
+- Prefer the shared workspace rules over model improvisation.
+- Before answering current-state questions, inspect the current-work files and latest relevant daily note.
+- If a prompt changes durable project understanding or reusable agent behavior, update the smallest correct canonical file.