feat: Implement GenAgent trait framework (unblock terraphim_kg_agents)

UPLIFT-2: Implement GenAgent trait framework

Source: medgemma-competition multi-agent integration plan
Priority: High (unblocks ~2,500 lines of disabled agent code)

Problem

terraphim_kg_agents has ~2,500 lines of agent code (worker, planning, coordination) that is ALL commented out in lib.rs because it references a non-existent terraphim_gen_agent crate and GenAgent<S> trait. The trait is conceptually similar to Erlang's gen_server but with typed state and async lifecycle hooks.

Proposed Change

Define GenAgent<S> trait in a new terraphim_gen_agent crate (or inside terraphim_agent_supervisor).

Minimum viable trait:

#[async_trait]
pub trait GenAgent<S: Send + Sync + 'static>: Send + Sync {
    async fn init(&self) -> Result<S>;
    async fn handle_call(&self, state: &mut S, msg: Box<dyn Any + Send>) -> Result<Box<dyn Any + Send>>;
    async fn handle_cast(&self, state: &mut S, msg: Box<dyn Any + Send>) -> Result<()>;
    async fn handle_info(&self, state: &mut S, info: SystemInfo) -> Result<()>;
    async fn terminate(&self, state: &mut S, reason: ExitReason) -> Result<()>;
}

Then uncomment and fix the three agent modules in terraphim_kg_agents.

Files to Change

• New crate or addition to crates/terraphim_agent_supervisor/
• crates/terraphim_kg_agents/src/lib.rs (uncomment modules)
• crates/terraphim_kg_agents/src/worker.rs
• crates/terraphim_kg_agents/src/planning.rs
• crates/terraphim_kg_agents/src/coordination.rs

Acceptance Criteria

• GenAgent<S> trait defined with async lifecycle hooks
• All three agent modules in terraphim_kg_agents uncommented and compiling
• At least one integration test demonstrating a GenAgent lifecycle (init -> handle_call -> terminate)
• Existing tests in terraphim_kg_agents pass

Start by reading crates/terraphim_kg_agents/src/lib.rs and the commented worker.rs, planning.rs, and coordination.rs modules, then inspect crates/terraphim_agent_supervisor/ for the proposed integration point. Define the trait and lifecycle types only after understanding those consumers. Done means all three modules compile, an integration test covers init → handle_call → terminate, and the existing terraphim_kg_agents tests pass.