Add bake preview_link task and bin/preview-link for ad-hoc link previews

Builds a link post via Pressa::LinkPost, builds the site for http://mudge:8000
(blog-server.service serves www/ straight off disk), prints the preview URL,
then deletes the local post file again. No git pull/commit/push at all --
used by the mudge dashboard's new 'Publish Beta' button to preview a post
without committing anything.

bake.rb

@@ -101,6 +101,49 @@ def new_link
   puts post.target_path
 end
 
+# Build a link post and preview it on the mudge blog server without touching
+# git: writes posts/YYYY/MM/slug.md, builds for http://mudge:8000 (the same
+# target blog-server.service serves straight out of www/), prints the
+# preview URL, then deletes the local file. Drives bin/preview-link.
+def preview_link
+  payload =
+    begin
+      JSON.parse($stdin.read)
+    rescue JSON::ParserError => e
+      abort "Error: invalid JSON payload on stdin: #{e.message}"
+    end
+
+  author = payload["author"] || Pressa::Config::SimpleToml.load_file("site.toml")["author"]
+  post =
+    begin
+      Pressa::LinkPost.build(
+        title: payload["title"],
+        link: payload["link"],
+        body: payload["body"],
+        tags: payload["tags"],
+        author:
+      )
+    rescue Pressa::LinkPost::Error => e
+      abort "Error: #{e.message}"
+    end
+
+  abort "Error: post already exists at #{post.target_path}" if File.exist?(post.target_path)
+
+  FileUtils.mkdir_p(File.dirname(post.target_path))
+  File.write(post.target_path, post.content)
+
+  year_month = post.target_path[%r{^posts/(\d{4}/\d{2})/}, 1]
+  slug = File.basename(post.filename, ".md")
+
+  begin
+    mudge
+  ensure
+    FileUtils.rm_f(post.target_path)
+  end
+
+  puts "http://mudge:8000/posts/#{year_month}/#{slug}/"
+end
+
 # Create a new draft in public/drafts/.
 # @parameter title_parts [Array] Optional title words; defaults to Untitled.
 def new_draft(*title_parts)

bin/preview-link

@@ -0,0 +1,39 @@
+#!/usr/bin/env bash
+#
+# Build a link post from a JSON payload on stdin and preview it on the mudge
+# blog server (http://mudge:8000, served by blog-server.service straight out
+# of www/) without touching git at all: no pull, no commit, no push. The post
+# file is deleted from posts/ again right after the preview build, so this
+# never leaves a trace in the repo or in git history.
+#
+#   ssh mudge '$HOME/samhuri.net/bin/preview-link' <<'JSON'
+#   {"title": "Magical Wristband", "link": "https://example.net/x",
+#    "body": "Optional commentary.", "tags": "gear, tech"}
+#   JSON
+#
+# Prints the preview URL on stdout, progress on stderr. Used by the
+# dashboard's "Publish Beta" button.
+
+set -euo pipefail
+
+# Non-interactive SSH sessions get a bare PATH; make sure rbenv is reachable.
+export PATH="$HOME/.rbenv/bin:$HOME/.rbenv/shims:$PATH"
+if command -v rbenv >/dev/null 2>&1; then
+  eval "$(rbenv init - bash)"
+fi
+
+# Run from the repo root regardless of where the script lives.
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+REPO="${SAMHURI_REPO:-$(cd "$SCRIPT_DIR/.." && pwd)}"
+cd "$REPO"
+
+payload="$(cat)"
+if [ -z "${payload//[[:space:]]/}" ]; then
+  echo "Error: empty payload on stdin" >&2
+  exit 1
+fi
+
+echo "==> Building preview" >&2
+# `bake mudge` logs its own build progress to stdout (not stderr), so only the
+# final line -- the preview URL `preview_link` puts last -- is what we want.
+printf '%s' "$payload" | bundle exec bake preview_link | tee /dev/stderr | tail -n 1