Runtime Scripts: add the first curated gameplay host API #319

This was generated by AI during triage.

Agent Brief

Category: enhancement
Summary: Expose the first small set of gameplay operations to Runtime Scripts.

Current behavior:
The Runtime Script host can execute Lua but has no gameplay contract.

Desired behavior:
Provide event payload access, read-only actor and target fields, healing, and chat output. Runtime Scripts never receive table handles, reducer context, SQL, or unrestricted entity mutation. Healing and chat use the same core gameplay operations and Gates as compiled behavior. All state-changing calls produce staged effects that commit only when the invocation succeeds.

Key interfaces:

• Event payload value
• Actor and target read views
• heal host operation
• send_chat host operation

Acceptance criteria:

• Scripts can inspect documented event, actor, and target fields.
• Missing actors or targets produce a defined script result, not a host panic.
• Heal and chat go through core operations and commit only after script success.
• A later script failure cannot undo effects committed by an earlier successful script.
• No raw table or arbitrary reducer access is reachable.
• Host-call tests cover invalid identifiers, refused operations, and fuel failure after staging.

Out of scope:

• Damage, items, experience, reputation, spawning, or scheduling
• Mutating hook inputs
• Raw data access