Summary

Wires the existing markdown pieces (markdown surface type, the secure markdown.load_file pipe handler, the marked + DOMPurify renderer) into a working feature and adds the missing entry points, mirroring the diff view.

Previously the render chain was dead: __wmux_setMarkdownContent dispatched a wmux:markdown-update CustomEvent that no component listened to, and MarkdownPane was mounted without a content prop — so markdown never rendered (always showed the empty placeholder).

Related Issue

Closes #54

Changes

• Fix render chain: persist content per surface via SurfaceRef.markdownContent, add a setMarkdownContent(surfaceId, content) store action (locates the owning pane by id across workspaces), and thread it into MarkdownPane via a content prop. Replaces the dead CustomEvent.
• One-shot CLI wmux markdown <file>: creates a markdown surface and loads the file in one step. Relative paths resolve against the terminal's cwd (path.resolve(process.cwd(), …)), not the Electron main-process cwd. wmux markdown set <id> --content/--file preserved; --file now also resolves relative paths.
• Manual UI entry point: new markdown:open-file IPC (native file picker filtered to allowed extensions, reusing the same extension-whitelist + 5 MB guards as the pipe handler) + a command-palette item "Open Markdown File…".
• Teach Claude: added a ## Markdown section to resources/claude-instructions.md (auto-injected into ~/.claude/CLAUDE.md by claude-context.ts).
• Tests: tests/unit/markdown-content.test.ts (4 tests) covering setMarkdownContent.

Acceptance criteria

• wmux markdown set <id> --content "# Hi" and --file <path> render in the pane
• wmux markdown <relative-or-absolute.md> opens a new markdown surface showing the file
• wmux markdown set <id> ... still works (no regression)
• Relative paths resolve against the terminal's cwd
• A manual UI entry point opens a markdown file into a view (command palette)
• resources/claude-instructions.md documents the command (lands in ~/.claude/CLAUDE.md on next launch)
• Non-markdown / oversized / missing files rejected with a clear error (guards preserved on both paths)

Test Plan

npm run dev
# from a wmux terminal pane:
wmux markdown README.md                      # opens a new markdown view
wmux markdown set <surfaceId> --content "# Hi"
# or: Command Palette → "Open Markdown File…"

• npm run build:main ✅ · npx vite build ✅ · npx vitest run tests/unit/markdown-content.test.ts ✅ (4/4)
• Pre-existing pty-manager test failures (node-pty native module unavailable in this env) are unrelated — they fail identically on clean master.

Note: dompurify was already declared in package.json but missing from node_modules; installed it (lockfile unchanged). PR pushed from fork kevmtt/wmux (no write access to upstream).