docs: add missing AGENTS.md (and README where missing)

AGENTS.md

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+# AGENTS.md
+
+Guidance for coding agents working in this repository.
+
+## What this is
+
+A Kotlin compiler plugin (`com.sourcegraph:semanticdb-kotlinc`) that emits
+[SemanticDB](https://scalameta.org/docs/semanticdb/specification.html) data for
+Kotlin sources, used with [scip-java](https://sourcegraph.github.io/scip-java)
+to produce [SCIP](https://github.com/sourcegraph/scip) indexes. See
+`README.md` for usage.
+
+## Layout
+
+- `build.sbt` — sbt build defining three projects: `kotlinc` (the plugin),
+  `minimized` (compile fixtures + snapshot generation), and `snapshotsRunner`
+  (runs scip-java).
+- `semanticdb-kotlinc/src/main/kotlin/com/sourcegraph/semanticdb_kotlinc/` —
+  main plugin sources (analyzer, registrars, SemanticDB builder/visitor).
+- `semanticdb-kotlinc/src/main/proto/` — protobuf definitions (Java codegen).
+- `semanticdb-kotlinc/src/test/kotlin/.../semanticdb_kotlinc/test/` — tests.
+- `semanticdb-kotlinc/minimized/` — fixtures and golden snapshots under
+  `minimized/src/generatedSnapshots/resources/`.
+- `project/` — sbt version (`build.properties`) and plugins (`plugins.sbt`).
+
+## Setup
+
+- Build tool: sbt (`sbt.version` pinned in `project/build.properties`).
+- JDK: 11 (used by CI).
+- Kotlin version is pinned in `build.sbt` (`V.kotlin`); each release supports a
+  single major Kotlin version (see the compatibility table in `README.md`).
+
+## Common commands
+
+Run from the repository root:
+
+```sh
+# Run the plugin's test suite
+sbt kotlinc/test
+
+# Regenerate the golden SemanticDB/SCIP snapshots
+sbt minimized/snapshots
+
+# Build the shaded fat-jar / publish locally
+sbt kotlinc/assembly
+sbt kotlinc/publishLocal
+```
+
+## Conventions
+
+- After changing plugin behavior, regenerate snapshots with
+  `sbt minimized/snapshots`. CI fails on snapshot drift via:
+
+  ```sh
+  git diff --exit-code semanticdb-kotlinc/minimized/src/generatedSnapshots
+  ```
+
+- Formatting for `.sbt`/Scala build files is handled by sbt-scalafmt
+  (`sbt scalafmtAll`).
+- `kotlin-stdlib` and `kotlin-compiler-embeddable` are `Provided` — they are
+  supplied by `kotlinc` at runtime and must not be bundled into the fat-jar.
+
+## CI
+
+`.github/workflows/ci.yml` runs `sbt kotlinc/test`, `sbt minimized/snapshots`,
+and the snapshot-drift check. `.github/workflows/release.yml` publishes via
+`sbt ci-release` on pushes to `main` and `v*` tags.