Clean up AGENTS.md

AGENTS.md

@@ -6,46 +6,26 @@ Rust framework for numerical problem solving. See README.md for usage examples.
 
 - **twine-core** (`crates/core`): Shared traits and types
 - **twine-solvers** (`crates/solvers`): Solver implementations, organized by problem type
+- **twine-observers** (`crates/observers`): Reusable `Observer` implementations
 
 ## Core Abstraction
 
-```
-Model::call(input) -> output
-Problem::input(x) -> model_input
-Problem::residuals|objective(input, output) -> metric
-Solver::solve(model, problem, bracket, config, observer) -> Solution
-```
+Three problem types: equation (root-finding), optimization, and ODE. All follow the same pattern:
 
-Solvers are generic over Model and Problem. Problems adapt solver variables (`x: [f64; N]`) 
-to model inputs and extract metrics from outputs.
+- `Model::call(input) -> Result<output, error>`
+- `Problem::input(x) -> model_input` — maps solver variables to model input
+- `Problem` extracts a metric from input/output (residuals, objective, or derivative)
 
-## Observer Pattern
-
-Solvers emit events; observers optionally return actions.
-
-```rust
-trait Observer<Event, Action> {
-    fn observe(&mut self, event: &Event) -> Option<Action>;
-}
+Solvers are generic over `Model` and `Problem`.
 
-impl<E, A> Observer<E, A> for () {
-    fn observe(&mut self, _: &E) -> Option<A> { None }
-}
-```
+## Observer Pattern
 
-Events expose solver state (current point, bracket, errors).
-Actions steer behavior (stop early, assume sign/worse for recovery).
+`Observer<E, A>` is defined in `twine-core`. Closures and `()` both implement it — `()` is the no-op observer. Solvers emit events; observers optionally return actions to steer behavior (stop early, assume worse, etc.).
 
 ## Solver Conventions
 
-- Public functions: `solve`/`minimize`/`maximize` with observer, `*_unobserved` without
-- Module structure: one file per concern (bracket, config, event, action, solution, error)
+- Public API: `solve`/`minimize`/`maximize` (with observer) and `*_unobserved` variants
 - Config validation at entry, not per-iteration
-- `Solution` contains: status, solver variable, objective/residual, snapshot (input+output), iters
-
-## Workflow notes
-
-Feature branches may include temporary workflow docs (e.g. TODO.md). Do not merge them to main.
 
 ## Testing