Add Mermaid architecture diagram to README

Add a Mermaid diagram to the README showing the autoloop flow. Something like:

flowchart TD
    subgraph "Pre-step"
        S[autoloop_scheduler.py] -->|reads| PD[".autoloop/programs/{name}/program.md"]
        S -->|reads| SF[("memory/autoloop branch<br/>{name}.md state file")]
        S -->|writes| AJ["/tmp/gh-aw/autoloop.json"]
    end

    subgraph "Agent Step"
        AJ -->|selected program| A1["Step 1-2: Read program,<br/>state file, target files,<br/>issue comments"]
        A1 --> A3["Step 3: Checkout autoloop/{name}<br/>fast-forward to main"]
        A3 --> A4["Step 4: Propose change,<br/>run evaluation command"]
        A4 -->|metric improved| A5a["Step 5a: Commit → push<br/>via create_pull_request /<br/>push_to_pull_request_branch"]
        A4 -->|no improvement| REJ["Reject: update state,<br/>no push"]
        A5a -->|CI green| A5c["Step 5c: Accept<br/>update state, post to issue"]
        A5a -->|CI red| A5b["Step 5b: Fix loop<br/>(up to 5 attempts)"]
        A5b -->|fixed| A5a
        A5b -->|exhausted| PAUSE["Pause program"]
    end

    subgraph "Safe Outputs Step"
        A5a -->|patch artifact| SO["Apply patch to remote branch<br/>Create/update PR<br/>Post comments to issue"]
    end

    subgraph "Persistence"
        A5c -->|writes| SF
        REJ -->|writes| SF
        SO -->|pushes| BR["autoloop/{name} branch"]
        SO -->|creates/updates| PR["Draft PR"]
        SO -->|comments on| ISS["Program issue<br/>[Autoloop: {name}]"]
    end

    subgraph "Supporting"
        EV["Evergreen (every 5m)"] -->|fixes CI / merges main| PR
        CI["CI workflow"] -->|runs on push| BR
    end

The diagram should cover:

• The three phases: scheduler pre-step → agent step → safe-outputs step
• What gets read (program def, state file, issue comments, target files)
• The accept/reject decision tree
• The CI fix loop
• How state persists (repo-memory branch, program issue, PR branch)
• How Evergreen and CI interact with the autoloop PRs