docs: add OPSX experimental workflow visibility to README (#460)

* docs: add OPSX experimental workflow visibility to README

Add a subtle banner near the top and an Experimental Features section
before Contributing to draw attention to the new OPSX workflow.

* docs: reframe OPSX messaging around fluid iteration

Update messaging to emphasize the core value proposition:
- No phases, just actions
- Dependencies are enablers, not gates
- Update artifacts as you learn during implementation

The previous "step-by-step artifact creation" framing incorrectly
suggested more bureaucracy. OPSX is about less rigidity, not more.

* docs: add architecture deep dive with ASCII diagrams

Add comprehensive comparison of standard vs OPSX workflow architecture:
- Philosophy: phases vs actions
- Component architecture diagrams
- Dependency graph model
- Information flow comparison
- Iteration model comparison
- Custom schema example

* docs: emphasize hackability and experimentation rationale

Add "Why We Built This" section explaining the meta-level motivation:
- Instructions were hardcoded, hard to improve
- Needed granular, testable artifacts
- Wanted to experiment with different workflows without code changes
- OPSX makes the instruction system itself hackable

Update README banner and experimental features section to highlight
schema-driven, hackable nature alongside fluid iteration benefits.

* docs: reframe hackability as user benefit, not just internal

OPSX isn't just for OpenSpec devs to experiment - it's for everyone:
- Teams can create workflows that match how they work
- Power users can tweak prompts to get better AI outputs
- Contributors can experiment without releases

Updated framing from "we needed" to "now anyone can".

* docs: add guidance on when to update vs. start fresh

Addresses a common question: when does "update as you learn" become
"this is different work"? Adds heuristics based on intent, scope
overlap, and completability to help users make the judgment call.

Author: TabishB

README.md

@@ -26,6 +26,10 @@
   Follow <a href="https://x.com/0xTab">@0xTab on X</a> for updates · Join the <a href="https://discord.gg/YctCnvvshC">OpenSpec Discord</a> for help and questions.
 </p>
 
+<p align="center">
+  <sub>🧪 <strong>New:</strong> <a href="docs/experimental-workflow.md">Experimental Workflow (OPSX)</a> — schema-driven, hackable, fluid. Iterate on workflows without code changes.</sub>
+</p>
+
 # OpenSpec
 
 OpenSpec aligns humans and AI coding assistants with spec-driven development so you agree on what to build before any code is written. **No API keys required.**
@@ -368,6 +372,44 @@ Run `openspec update` whenever someone switches tools so your agents pick up the
 2. **Refresh agent instructions**
    - Run `openspec update` inside each project to regenerate AI guidance and ensure the latest slash commands are active.
 
+## Experimental Features
+
+<details>
+<summary><strong>🧪 OPSX: Fluid, Iterative Workflow</strong> (Claude Code only)</summary>
+
+**Why this exists:**
+- Standard workflow is locked down — you can't tweak instructions or customize
+- When AI output is bad, you can't improve the prompts yourself
+- Same workflow for everyone, no way to match how your team works
+
+**What's different:**
+- **Hackable** — edit templates and schemas yourself, test immediately, no rebuild
+- **Granular** — each artifact has its own instructions, test and tweak individually
+- **Customizable** — define your own workflows, artifacts, and dependencies
+- **Fluid** — no phase gates, update any artifact anytime
+
+```
+You can always go back:
+
+  proposal ──→ specs ──→ design ──→ tasks ──→ implement
+     ▲           ▲          ▲                    │
+     └───────────┴──────────┴────────────────────┘
+```
+
+| Command | What it does |
+|---------|--------------|
+| `/opsx:new` | Start a new change |
+| `/opsx:continue` | Create the next artifact (based on what's ready) |
+| `/opsx:ff` | Fast-forward (all planning artifacts at once) |
+| `/opsx:apply` | Implement tasks, updating artifacts as needed |
+| `/opsx:archive` | Archive when done |
+
+**Setup:** `openspec artifact-experimental-setup`
+
+[Full documentation →](docs/experimental-workflow.md)
+
+</details>
+
 ## Contributing
 
 - Install dependencies: `pnpm install`