docs(domains): add README overview for physics kernels

Summarize continuum/heat/darcy/thermo_elastic kernels with pointers to AGENTS,
extension contract, tests, and thermo-elastic walkthrough.

src/domains/README.md

@@ -0,0 +1,22 @@
+# src/domains/
+
+Physics kernels that plug into the shared assembler infrastructure. Each
+subdirectory implements an `AbstractKernel` for one discipline or coupled
+problem.
+
+For architecture, invariants, and vocabulary, read `AGENTS.md` in the
+repository root. For the microkernel checklist when adding a kernel, read
+`juliafem.github.io/docs/developer-guide/kernel_extension_contract.md` in the
+same checkout.
+
+## Layout
+
+| Directory | Kernel(s) | Tests |
+|-----------|-----------|-------|
+| `continuum/` | `ContinuumKernel`, small-strain mechanics helpers | `test/domains/continuum/` |
+| `heat/` | `HeatKernel` | `test/domains/heat/` |
+| `darcy/` | `DarcyPotentialKernel` (primal potential); `DarcyMixedRT0P0Kernel` (Tet4 RT₀–P₀ H(div)) | `test/domains/darcy/` |
+| `thermo_elastic/` | `ThermoElasticKernel` (multi-field `u` + `T`) | `test/domains/thermo_elastic/` |
+
+Narrative walkthrough for thermo-elasticity: `docs/src/thermo_elastic_walkthrough.md`
+in the package Documenter tree.