docs: add project-config demo guide (Fission-AI#521)

Add a quick-reference demo guide for the project-config feature
(openspec/config.yaml). This consolidates the demo walkthrough
into a standalone document that's easier to use when presenting
the feature.

Includes:
- Summary of what project config does
- 6 numbered demo scenarios
- Quick all-in-one demo script
- Key points to emphasize

Author: TabishB

docs/project-config-demo.md

@@ -0,0 +1,205 @@
+# Project Config Demo Guide
+
+A quick-reference guide for demonstrating the `openspec/config.yaml` feature.
+
+## Summary: What Project Config Does
+
+The feature adds `openspec/config.yaml` as a lightweight customization layer that lets teams:
+
+- **Set a default schema** - New changes automatically use this schema instead of having to specify `--schema` every time
+- **Inject project context** - Shared context (tech stack, conventions) shown to AI when creating any artifact
+- **Add per-artifact rules** - Custom rules that only apply to specific artifacts (e.g., proposal, specs)
+
+## Demo Walkthrough
+
+### Demo 1: Interactive Setup (Recommended Entry Point)
+
+The easiest way to demo is through the experimental setup command:
+
+```bash
+openspec artifact-experimental-setup
+```
+
+After creating skills/commands, it will prompt:
+
+```
+━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+📋 Project Configuration (Optional)
+
+Configure project defaults for OpenSpec workflows.
+
+? Create openspec/config.yaml? (Y/n)
+```
+
+Walk through:
+
+1. **Select schema** - Shows available schemas with their artifact flows
+2. **Add context** - Opens editor for multi-line project context (tech stack, conventions)
+3. **Add rules** - Checkbox to select artifacts, then line-by-line rule entry
+
+This creates `openspec/config.yaml` with the user's choices.
+
+### Demo 2: Manual Config Creation
+
+Show that users can create the config directly:
+
+```bash
+cat > openspec/config.yaml << 'EOF'
+schema: spec-driven
+
+context: |
+  Tech stack: TypeScript, React, Node.js, PostgreSQL
+  API style: RESTful, documented in docs/api.md
+  Testing: Jest + React Testing Library
+  We value backwards compatibility for all public APIs
+
+rules:
+  proposal:
+    - Include rollback plan
+    - Identify affected teams and notify in #platform-changes
+  specs:
+    - Use Given/When/Then format
+    - Reference existing patterns before inventing new ones
+EOF
+```
+
+### Demo 3: Effect on New Changes
+
+Show that creating a new change now uses the default schema:
+
+```bash
+# Before config: had to specify schema
+openspec new change my-feature --schema spec-driven
+
+# After config: schema is automatic
+openspec new change my-feature
+# Automatically uses spec-driven from config
+```
+
+### Demo 4: Context and Rules Injection
+
+The key demo moment - show how instructions are enriched:
+
+```bash
+# Get instructions for an artifact
+openspec instructions proposal --change my-feature
+```
+
+Output shows the XML structure:
+
+```xml
+<context>
+Tech stack: TypeScript, React, Node.js, PostgreSQL
+API style: RESTful, documented in docs/api.md
+...
+</context>
+
+<rules>
+- Include rollback plan
+- Identify affected teams and notify in #platform-changes
+</rules>
+
+<template>
+[Schema's built-in proposal template]
+</template>
+```
+
+Key points to highlight:
+
+- **Context** appears in ALL artifacts (proposal, specs, design, tasks)
+- **Rules** ONLY appear for the matching artifact (proposal rules only in proposal instructions)
+
+### Demo 5: Precedence Override
+
+Show the schema resolution order:
+
+```bash
+# Config sets schema: spec-driven
+
+# 1. CLI flag wins
+openspec new change feature-a --schema tdd  # Uses tdd
+
+# 2. Change metadata wins over config
+# (if .openspec.yaml in change directory specifies schema)
+
+# 3. Config is used as default
+openspec new change feature-b  # Uses spec-driven from config
+
+# 4. Hardcoded default (no config)
+# Would fall back to spec-driven anyway
+```
+
+### Demo 6: Validation and Error Handling
+
+Show graceful error handling:
+
+```bash
+# Create config with typo
+echo "schema: spec-drivne" > openspec/config.yaml
+
+# Try to use it - shows fuzzy matching suggestions
+openspec new change test
+# Schema 'spec-drivne' not found
+# Did you mean: spec-driven (built-in)
+```
+
+```bash
+# Unknown artifact ID in rules - warns but doesn't halt
+cat > openspec/config.yaml << 'EOF'
+schema: spec-driven
+rules:
+  testplan:  # Schema doesn't have this
+    - Some rule
+EOF
+
+openspec instructions proposal --change test
+# ⚠️ Unknown artifact ID in rules: "testplan". Valid IDs for schema "spec-driven": ...
+# (continues working)
+```
+
+## Quick Demo Script
+
+Here's a quick all-in-one demo:
+
+```bash
+# 1. Show there's no config initially
+cat openspec/config.yaml 2>/dev/null || echo "No config exists"
+
+# 2. Create a simple config
+cat > openspec/config.yaml << 'EOF'
+schema: spec-driven
+context: |
+  This is a demo project using React and TypeScript.
+  We follow semantic versioning.
+rules:
+  proposal:
+    - Include migration steps if breaking change
+EOF
+
+# 3. Show the config
+cat openspec/config.yaml
+
+# 4. Create a change (uses default schema from config)
+openspec new change demo-feature
+
+# 5. Show instructions with injected context/rules
+openspec instructions proposal --change demo-feature | head -30
+
+# 6. Show that specs don't have proposal rules
+openspec instructions specs --change demo-feature | head -30
+```
+
+## What to Emphasize in Demo
+
+- **Low friction** - Teams can customize without forking schemas
+- **Shared context** - Everyone on the team gets the same project knowledge
+- **Per-artifact rules** - Targeted guidance where it matters
+- **Graceful failures** - Typos warn, don't break workflow
+- **Team sharing** - Just commit `openspec/config.yaml` and everyone benefits
+
+## Related Documentation
+
+- [Experimental Workflow Guide](./experimental-workflow.md) - Full user guide with config section
+- [Project Config Proposal](../openspec/changes/project-config/proposal.md) - Original design proposal
+- [Project Config Design](../openspec/changes/project-config/design.md) - Technical implementation details