Add Mermaid architecture diagram to README

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

```mermaid
flowchart TD
subgraph "Pre-step"
S[autoloop_scheduler.py] -->|reads| PD[".autoloop/programs/{name}/program.md"]
S -->|reads| SF[("memory/autoloop branch
{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,
state file, target files,
issue comments"]
A1 --> A3["Step 3: Checkout autoloop/{name}
fast-forward to main"]
A3 --> A4["Step 4: Propose change,
run evaluation command"]
A4 -->|metric improved| A5a["Step 5a: Commit → push
via create_pull_request /
push_to_pull_request_branch"]
A4 -->|no improvement| REJ["Reject: update state,
no push"]
A5a -->|CI green| A5c["Step 5c: Accept
update state, post to issue"]
A5a -->|CI red| A5b["Step 5b: Fix loop
(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
Create/update PR
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
[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