docs: add CLAUDE.md pointing at the standards document

Agent sessions did not pick up docs/STANDARDS.md automatically. CLAUDE.md is
loaded every session, so it links the standards, architecture, and test docs,
repeats the few rules most often broken, and records the CGO/MSYS2 build
prerequisite and the commit-to-main convention.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CLAUDE.md

@@ -0,0 +1,53 @@
+# GoSentry — instructions for Claude Code
+
+Cross-platform desktop scheduler (Go + Fyne GUI). Single process: GUI,
+application service, scheduler, storage, and command runner in one binary.
+
+## Read before changing code
+
+- [docs/STANDARDS.md](docs/STANDARDS.md) — **required.** Code-quality rules and
+  the list of intentional behavior. Do not "fix" anything listed there as
+  intentional; if a change contradicts it, update the document in the same commit.
+- [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md) — package contracts and event flow.
+- [docs/TESTS.md](docs/TESTS.md) — test layout and conventions.
+- [docs/ROADMAP.md](docs/ROADMAP.md) — deliberately out of scope.
+
+## Key rules (full list in STANDARDS.md)
+
+- `src/app.Service` is the sole owner of job and runtime state; the UI reads it
+  through typed events, never through shared mutable state.
+- User-facing errors go to `dialog.ShowError` or a History event — never a silent
+  `return`.
+- Pure helpers get a unit test in the same package; fixes of severity ≥ medium get
+  a regression test.
+- UI view constructors accept an injected `*app.Service`; `app.Open()` is called
+  only from `run.go`.
+- Off-main-thread widget updates must go through `fyne.Do` (Fyne v2.6.x).
+
+## Build and test
+
+CGO is required — the Fyne GUI links native libraries. On Windows the toolchain
+is MSYS2 UCRT64; the default shell environment here has CGO off, so set it
+explicitly:
+
+```powershell
+$env:Path = 'C:\msys64\ucrt64\bin;' + $env:Path; $env:CGO_ENABLED = '1'
+```
+
+Then:
+
+```powershell
+scripts\test.bat
+```
+
+which runs `go vet ./...` and `go test -race ./...`. Release binaries come from
+`scripts\build-windows.bat` / `scripts/build-linux.sh` — see
+[docs/DEVELOPMENT.md](docs/DEVELOPMENT.md).
+
+## Repository conventions
+
+- Commit directly to `main`; do not create feature branches.
+- Notable changes get a [docs/CHANGELOG.md](docs/CHANGELOG.md) entry under the
+  current version.
+- The window/taskbar icon comes from the `gosentry.ico` PE resource — regenerate
+  it from the PNGs whenever an icon changes, not just the embedded asset.