Require explicit tool use and add inventory limits

[justinmadison]

Summary

Disable auto-collection, require explicit pickup_item/collect tool calls, and add inventory capacity limits to make tool selection a real decision for the LLM.

Motivation

Currently _check_resource_collection() in scripts/foraging.gd auto-collects resources when the agent is within COLLECTION_RADIUS (2.0 units). The agent never needs to explicitly decide to pick something up. The existing pickup_item, drop_item, and use_item tools are registered but never needed.

Requiring explicit tool use means the agent must:

• Choose when to collect — maybe it should scout first before committing
• Manage inventory — with limited slots, it must decide what to carry
• Use items strategically — eating an apple to restore health, using a torch to see further

Implementation Details

Disable Auto-Collection

• Remove or gate the auto-collection in _check_resource_collection() in scripts/foraging.gd
• Resources stay in the world until the agent explicitly calls pickup_item or collect
• Agent must be within COLLECTION_RADIUS to pick up (same proximity check, just not automatic)

Inventory Limits

• Add MAX_INVENTORY_SLOTS = 5 to foraging scene constants
• pickup_item fails if inventory is full (return error in tool result)
• Agent must call drop_item to make room
• Add inventory count to observation: "inventory_slots_used": 3, "inventory_capacity": 5

Item Weight (Optional)

• Heavier items (stone=3, wood=2, apple=1) reduce movement speed proportionally
• Adds another trade-off dimension: carry fewer heavy items or more light ones
• current_weight and max_weight in observations

Item Use

• use_item("apple") — restores 20 health (currently no way to regain health)
• use_item("torch") — temporarily increases perception radius by 5 units (ties into #perception-radius issue)
• Crafted items have uses too (shelter provides a safe zone, etc.)

Starter Agent Updates

• Add pickup_item, drop_item, use_item to VALID_TOOLS
• Update decision prompt to explain inventory management
• Update system prompt with new tool definitions

Impact on RAG Memory

• "Inventory fills up fast — should drop stone to pick up apple when health is low"
• "Using torch before exploring east quadrant reveals more resources"
• Strategy learning across episodes about optimal inventory management

Acceptance Criteria

• Auto-collection disabled; explicit pickup_item/collect required
• Inventory capped at 5 slots; pickup_item returns error when full
• drop_item functional — agent can free inventory space
• use_item functional for at least apples (health restore)
• Observation payload includes inventory capacity info
• Starter agent VALID_TOOLS and prompts updated