fix: use scratchpad/architecture.md as cache source of truth for architecture diagram

[Copilot]

The architecture diagram workflow could incorrectly noop even when scratchpad/architecture.md was empty or stale, because the noop gate only checked last_commit in the cache — not whether the diagram file actually existed with content.

Changes

• Removed last_diagram from architecture-state.json — cache now stores only last_commit and package_map; the diagram lives in scratchpad/architecture.md
• Noop condition now gates on scratchpad/architecture.md — requires the file to exist with diagram content in addition to no Go file changes since last_commit
• Updated cache write — architecture-state.json no longer includes last_diagram field

Before

{
  "last_commit": "<sha>",
  "last_diagram": "<full ASCII diagram>",
  "package_map": { ... }
}

After

{
  "last_commit": "<sha>",
  "package_map": { ... }
}

The noop path now skips only when: no .go changes since last_commit AND scratchpad/architecture.md exists with diagram content.

[Copilot]

Pull request overview

Updates the daily architecture diagram generator workflow spec so the cache no-op behavior depends on the presence of a valid diagram in scratchpad/architecture.md, rather than relying on a cached last_diagram field.

Changes:

• Remove last_diagram from the documented architecture-state.json schema (cache now stores last_commit and package_map only).
• Update the no-op gate to require scratchpad/architecture.md to exist and contain diagram content.
• Update the documented cache write example to match the new schema.

Show a summary per file

File	Description
.github/workflows/daily-architecture-diagram.md	Aligns the workflow spec’s cache schema and no-op logic with scratchpad/architecture.md as the diagram source of truth.

Copilot's findings

Tip

Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

• Files reviewed: 1/1 changed files
• Comments generated: 0