Add antigravity support and generic overlays structure

overlays/antigravity/plan-product.md

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+# Plan Product
+
+Establish foundational product documentation through an interactive conversation. Creates mission, roadmap, and tech stack files in `agent-os/product/`.
+
+## Important Guidelines
+
+- **Always ask the user** when you need input or key decisions.
+- **Keep it lightweight** — gather enough to create useful docs without over-documenting
+- **One question at a time** — don't overwhelm with multiple questions
+
+## Process
+
+### Step 1: Check for Existing Product Docs
+
+Check if `agent-os/product/` exists and contains any of these files:
+- `mission.md`
+- `roadmap.md`
+- `tech-stack.md`
+
+**If any files exist**, ask the user:
+
+```
+I found existing product documentation:
+- mission.md: [exists/missing]
+- roadmap.md: [exists/missing]
+- tech-stack.md: [exists/missing]
+
+Would you like to:
+1. Start fresh (replace all)
+2. Update specific files
+3. Cancel
+
+(Choose 1, 2, or 3)
+```
+
+If option 2, ask which files to update and only gather info for those.
+If option 3, stop here.
+
+**If no files exist**, proceed to Step 2.
+
+### Step 2: Gather Product Vision (for mission.md)
+
+Ask the user:
+
+```
+Let's define your product's mission.
+
+**What problem does this product solve?**
+
+(Describe the core problem or pain point you're addressing)
+```
+
+After they respond, ask the user:
+
+```
+**Who is this product for?**
+
+(Describe your target users or audience)
+```
+
+After they respond, ask the user:
+
+```
+**What makes your solution unique?**
+
+(What's the key differentiator or approach?)
+```
+
+### Step 3: Gather Roadmap (for roadmap.md)
+
+Ask the user:
+
+```
+Now let's outline your development roadmap.
+
+**What are the must-have features for launch (MVP)?**
+
+(List the core features needed for the first usable version)
+```
+
+After they respond, ask the user:
+
+```
+**What features are planned for after launch?**
+
+(List features you'd like to add in future phases, or say "none yet")
+```
+
+### Step 4: Establish Tech Stack (for tech-stack.md)
+
+First, check if `agent-os/standards/global/tech-stack.md` exists.
+
+**If the tech-stack standard exists**, read it and ask the user:
+
+```
+I found a tech stack standard in your standards:
+
+[Summarize the key technologies from global/tech-stack.md]
+
+Does this project use the same tech stack, or does it differ?
+
+1. Same as standard (use as-is)
+2. Different (I'll specify)
+
+(Choose 1 or 2)
+```
+
+If they choose option 1, use the standard's content for tech-stack.md.
+If they choose option 2, proceed to ask them to specify (see below).
+
+**If no tech-stack standard exists** (or they chose option 2 above), ask the user:
+
+```
+**What technologies does this project use?**
+
+Please describe your tech stack:
+- Frontend: (e.g., React, Vue, vanilla JS, or N/A)
+- Backend: (e.g., Rails, Node, Django, or N/A)
+- Database: (e.g., PostgreSQL, MongoDB, or N/A)
+- Other: (hosting, APIs, tools, etc.)
+```
+
+### Step 5: Generate Files
+
+Create the `agent-os/product/` directory if it doesn't exist.
+
+Generate each file based on the information gathered:
+
+#### mission.md
+
+```markdown
+# Product Mission
+
+## Problem
+
+[Insert what problem this product solves - from Step 2]
+
+## Target Users
+
+[Insert who this product is for - from Step 2]
+
+## Solution
+
+[Insert what makes the solution unique - from Step 2]
+```
+
+#### roadmap.md
+
+```markdown
+# Product Roadmap
+
+## Phase 1: MVP
+
+[Insert must-have features for launch - from Step 3]
+
+## Phase 2: Post-Launch
+
+[Insert planned future features - from Step 3, or "To be determined" if they said none yet]
+```
+
+#### tech-stack.md
+
+```markdown
+# Tech Stack
+
+[Organize the tech stack information into logical sections]
+
+## Frontend
+
+[Frontend technologies, or "N/A" if not applicable]
+
+## Backend
+
+[Backend technologies, or "N/A" if not applicable]
+
+## Database
+
+[Database choice, or "N/A" if not applicable]
+
+## Other
+
+[Other tools, hosting, services - or omit this section if nothing mentioned]
+```
+
+### Step 6: Confirm Completion
+
+After creating all files, output to user:
+
+```
+✓ Product documentation created:
+
+  agent-os/product/mission.md
+  agent-os/product/roadmap.md
+  agent-os/product/tech-stack.md
+
+Review these files to ensure they accurately capture your product vision.
+You can edit them directly or run /plan-product again to update.
+```
+
+## Tips
+
+- If the user provides very brief answers, that's fine — the docs can be expanded later
+- If they want to skip a section, create the file with a placeholder like "To be defined"
+- The `/shape-spec` command will read these files when planning features, so having them populated helps with context