Add AGENTS guidance referencing SSOT

AGENTS.md

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+# AGENTS (Repository Guidance)
+
+## Repository Intent
+- The `web/desktop` and `web/mobile` folders host the live HUD demo (Desktop and mobile). Keep everything that should be served from a webserver inside those folders.
+- `docs/game_rulebook.md` is the **Single Source of Truth (SSOT)** for Galaxy Forge’s game logic, builder system, resources, and requirements. Any UI work, game behavior, or rules must align with — not contradict — that document unless a new decision is logged.
+- `docs/` stores documentation while `planning/` holds sketches/notes that are not meant for deployment.
+
+## Key Instructions for Agents
+1. **Reference SSOT first.** When implementing features or answering questions related to game balance, resources, blueprints, buildings, or world generation, open `docs/game_rulebook.md` and confirm compliance. Note any missing detail needs a Decision Log entry (see section 13).
+2. **Respect folder intent.** Changes to web assets go in `web/desktop` or `web/mobile` only. Use `planning/` for drafts and `docs/` for textual explanations.
+3. **Document assumptions.** If you must deviate from `docs/game_rulebook.md`, append an entry to the Decision Log (section 13) explaining the why/when.
+4. **Keep mobile/desktop aligned.** When adjusting UI/partials, mirror updates in both `web/desktop` and `web/mobile` unless the change is explicitly mobile-only or desktop-only and documented.
+
+## When touching docs
+- Expand `docs/game_rulebook.md` if you update core rules, flow, or systems described there. Keep numbering/sections intact; append entries in the Decision Log (section 13) with dates.
+- The root `README.md` should mention where to find the SSOT and purpose of each folder.
+
+## Decision Log Reminder
+- Section 13 of `docs/game_rulebook.md` is the Decision Log. Add a dated entry whenever:
+  - You introduce a new rule variant or exception.
+  - The implementation deviates from the documented defaults.