docs: add CONTRIBUTING (dev setup, claim-first, format-is-API)

Author: GabrielBBaldez

CONTRIBUTING.md

@@ -0,0 +1,59 @@
+# Contributing to stacktale for VS Code
+
+Thanks for stopping by the campfire. 🔥
+
+This is the VS Code / Cursor / Windsurf editor extension for
+[**stacktale**](https://github.com/stacktale/stacktale). The library lives in the main
+repo; this repo is just the extension that surfaces its `errors-ai.log` in the editor.
+
+New here? The [`good first issue`](https://github.com/stacktale/stacktale-vscode/labels/good%20first%20issue)
+label marks issues written to be picked up cold — each one names the files to touch and how
+to verify.
+
+## Claim the issue before you start
+
+**Comment on the issue saying you'd like to take it, and wait for a reply before writing
+code.** A short "I'd like to work on this" is enough — no need to restate the issue or
+explain your plan. This is the only thing standing between you and someone else spending an
+evening on the same fix, and it lets us tell you upfront if an issue is already half-done or
+narrower than it looks. If nobody replies in a day or two, open the PR anyway.
+
+Two exceptions, where you should just open the PR: an obvious typo or broken link, and
+anything already assigned to you.
+
+## Build & test
+
+```bash
+npm install
+npm run compile   # tsc → out/
+npm test          # compile + run the parser tests (node --test)
+```
+
+Press **F5** in VS Code to launch an Extension Development Host with the extension loaded.
+Open a project that has an `errors-ai.log` and the **stacktale** view appears in the
+activity bar. CI runs `compile` + `test` on every push.
+
+## Where the pieces live
+
+- **`src/stParser.ts`** — pure parser for the `errors-ai.log` (`st/1`) format. No VS Code
+  API here, so it's unit-testable in plain Node. This mirrors the parser in the
+  [JetBrains plugin](https://github.com/stacktale/stacktale-intellij); keep them in step.
+- **`src/extension.ts`** — the VS Code glue: the tree view, the file watcher, the commands.
+- **`src/test/parser.test.ts`** — `node:test` cases for the parser.
+
+## The report format is a public API
+
+The `errors-ai.log` format (`st/1`, and the opt-in `st-json/1`) is specified in the main
+repo's [docs/FORMAT.md](https://github.com/stacktale/stacktale/blob/main/docs/FORMAT.md).
+Parse defensively — a malformed or partially-written line must never crash the view — and
+when in doubt about a field, check FORMAT.md rather than guessing.
+
+## Working style
+
+- New parsing behavior arrives with the test that demanded it (`src/test/`).
+- Commits: conventional prefixes (`feat:`, `fix:`, `docs:`, `chore:`, `test:`), imperative
+  mood, reference issues (`Closes #N`).
+- One logical change per PR; explain the *why*, link the issue.
+
+By taking part you agree to the main repo's
+[Code of Conduct](https://github.com/stacktale/stacktale/blob/main/CODE_OF_CONDUCT.md).