feat(ci): add Graph() to visualize CI pipeline as Mermaid diagram (#126)

Adds a Ci.Graph() Dagger function that emits a Mermaid flowchart showing
both the Dagger Check pipeline (toolchain → pubGetLayer → parallel steps)
and the Codeberg CI job dependencies (check → build-linux / deploy-playstore
→ publish-website).

Usage: dagger call -m ci --source=. graph
       task ci-graph

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Taskfile.yml

@@ -189,6 +189,11 @@ tasks:
     cmds:
       - dagger call --progress=plain -q -m ci --source=. test-sync-reliability
 
+  ci-graph:
+    desc: Print a Mermaid diagram of the CI pipeline — paste into mermaid.live or any Markdown renderer
+    cmds:
+      - dagger call --progress=plain -q -m ci --source=. graph
+
   stalwart:
     desc: Start a Stalwart instance for local development (via Dagger)
     cmds: