Set up dev environment: AGENTS.md + toolchain notes

AGENTS.md

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+# AGENTS.md
+
+## Cursor Cloud specific instructions
+
+This repository is a study collection of standalone C/C++ algorithm implementations
+for physical-design / EDA topics (Steiner routing, MST/DSU, LCA variants, etc.).
+It is **not** a single buildable application: there is no build system, package
+manager, dependency manifest, test suite, or lint config. Each source file under
+`Steiner Routing/` (and its subdirectories) contains its own `main()` and is
+compiled and run independently.
+
+### Toolchain
+- Only a C/C++ toolchain is required. `g++`, `gcc`, `make`, and `clang++` are
+  preinstalled on the base image; there is nothing to install, so the startup
+  update script is just a toolchain sanity check.
+
+### Building / running a program
+- Compile a single file and run it, e.g.:
+  - `g++ -std=c++17 -O2 -o /tmp/prog "Steiner Routing/kruskals_mst.cpp" && /tmp/prog`
+- `Steiner Routing/dsu.cpp`, `kruskals_mst.cpp`, and `lca_tarjans/lca.cpp` print
+  to stdout directly.
+- `sparse_table.cpp` reads from **stdin** (RMQ queries), e.g.
+  `printf "5\n2 4 3 1 5\n2\n0 4\n1 3\n" | /tmp/sparse`.
+
+### File-I/O programs (gotcha)
+- `lca1/lca.cpp`, `lca_dp/lca_dp.cpp`, `lca_timer/lca_timer.cpp`, and
+  `euler_tours.cpp` use `freopen("inputs.txt", ...)` / `freopen("outputs.txt", ...)`.
+  They must be compiled and run **from inside their own directory** so the relative
+  `inputs.txt`/`outputs.txt` paths resolve. Running them overwrites the committed
+  `outputs.txt`; `git checkout -- <path>` to restore if you don't intend to change it.
+- The compiler emits harmless `-Wunused-result` warnings for the `freopen` calls.
+
+### Other notes
+- `Steiner Routing/lca_dp/a1.c` uses `sin`/`cos`; compile with `gcc a1.c -lm`.
+- `Convex Hull/`, `GPU - STA/`, and `Register Clustering/` contain only PDFs /
+  placeholder files — no code to build.