Add CLAUDE.md and ignore mise.toml

Document the codebase architecture and build commands for future
Claude Code sessions; mise.toml is a local tool-version file that
shouldn't be tracked.

Author: mapitman

.gitignore

@@ -12,3 +12,4 @@ parts/
 package/*
 *.deb
 mdview.1
+mise.toml

CLAUDE.md

@@ -0,0 +1,53 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## What this is
+
+`mdview` is a single-binary Go CLI that converts a markdown file to styled HTML and opens it in the
+default browser. It's essentially one file, `main.go`, plus a handful of embedded assets. There are
+no other Go source files and no test files currently in the repo.
+
+## Build & run
+
+Requires Go (see `go.mod`/`mise.toml` for version) and, for packaging targets, `just` and `pandoc`.
+
+- Quick local build: `go build` (produces `./mdview`)
+- Run directly without building: `go run . <filename.md>`
+- Full cross-platform build via `just` (uses `justfile`, the project's task runner — replaced the old Makefile):
+  - `just` or `just linux` — Linux amd64/arm64/i386 builds
+  - `just all` — linux + windows + darwin + freebsd
+  - `just deb` — build `.deb` packages (requires a prior linux build)
+  - `just snap` — build snap package via `snapcraft`
+  - `just clean` — remove build artifacts
+  - Pass `VERSION=x.y.z` as an env var to any target to stamp the version (embedded via `-ldflags -X main.appVersion=...`); CI derives this from the previous git tag.
+  - `just manpage` regenerates `mdview.1` from `mdview.1.md` via pandoc.
+- There is no test suite and no lint target configured; `go vet ./...` and `gofmt` are reasonable sanity checks before committing.
+
+## Architecture
+
+Everything happens in `main()` in `main.go`, in a straight-line pipeline:
+
+1. **Flag parsing** — `-o`, `-v/-version`, `-h/-help`, `-b/-bare`. First positional arg is the input markdown file.
+2. **Image inlining** (`processMarkdownImages` → `processHTMLImages`/markdown image regex + `imageToDataURI`) — rewrites relative image references (both `![]()`  markdown syntax and raw `<img src=...>` HTML) into base64 `data:` URIs *before* markdown parsing, so the output HTML is fully self-contained/offline-viewable. This includes path-traversal guards (caps `..` traversal depth) and a 10MB per-image size cap.
+3. **Markdown parsing** via Goldmark, configured with:
+   - `extension.GFM` (tables, task lists, strikethrough, etc.)
+   - `extension.Typographer` (smart quotes/dashes)
+   - `go.abhg.dev/goldmark/mermaid` with `NoScript: true` (diagram rendering is handled ourselves)
+   - `html.WithUnsafe()` (raw HTML passthrough is required for the image-tag handling above)
+4. **Title extraction** (`getTitleFromAST`/`extractText`) — walks the parsed AST for the first H1 to use as the `<title>`.
+5. **Mermaid script embedding** (`embedMermaidScript`) — only injects the embedded `mermaid.min.js` + an init script (with light/dark theme detection via `prefers-color-scheme`) if the rendered HTML actually contains a mermaid block, keeping non-diagram output lean.
+6. **Templating & output** — `template.html` and `github-markdown.css` are embedded via `//go:embed` and combined with the rendered content via `fmt.Fprintf`. `-bare` skips the CSS. Output goes to `-o <path>` if given, otherwise a random temp filename (see below).
+7. **Launch** — opens the resulting HTML file via `github.com/pkg/browser`.
+
+### Temp file / Snap sandboxing (`getTempDir`, `isSnap`)
+
+Output location resolution order: `MDVIEW_DIR` env var → (if running as a Snap, per `SNAP_USER_COMMON` env var) `~/mdview-temp` → OS default temp dir. This exists because Snap-sandboxed browsers (e.g. Snap Firefox) can't read from `/tmp`, only from the user's home directory — `check()` also prints a Snap-specific hint on file errors for this reason.
+
+### Packaging
+
+`justfile`, `control` (Debian control file template, `VERSION` substituted at build time), `mdview.1.md`
+(man page source) and `snap/` all support the release pipeline in `.github/workflows/release.yml`,
+which triggers on tag push, builds all platform targets + deb packages via `just`, and publishes a
+draft GitHub release with `softprops/action-gh-release`. `.github/workflows/build.yml` runs the same
+`just all` build on every push as a CI check (no tests to run).