Add Excalidraw Onboarding Guide with setup, features, and workflow de…

EXCALIDRAW_ONBOARDING.md

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+# Excalidraw Onboarding Guide
+
+<div align="center">
+  <p>
+    <strong>Virtual whiteboard for sketching hand-drawn like diagrams.</strong><br/>
+    Collaborative and end-to-end encrypted.
+  </p>
+</div>
+
+[Upstream repository](https://github.com/excalidraw/excalidraw) • [Docs](https://docs.excalidraw.com) • [Contributing guide](https://docs.excalidraw.com/docs/introduction/contributing)
+
+## What New Employees Should Know First
+
+Excalidraw is a TypeScript-first monorepo. During onboarding, focus on understanding the app runtime (`excalidraw-app`), shared packages (`packages`), and contribution workflow (PR quality + tests + CI checks).
+
+### Core Features (product context)
+
+- Infinite hand-drawn style canvas
+- Real-time collaboration and encrypted sharing
+- Rich drawing tools and shape libraries
+- Export to PNG, SVG, clipboard, and `.excalidraw` JSON
+- Dark mode, i18n, and local-first behavior
+
+## Repository Structure (Practical View)
+
+```text
+packages/            # Reusable editor modules and utilities
+excalidraw-app/      # Main application (excalidraw.com)
+dev-docs/            # Docusaurus technical docs
+examples/            # Integration examples
+scripts/             # Tooling, build and release scripts
+.github/             # CI and contribution automation
+```
+
+### Diagram: Codebase Orientation
+
+```mermaid
+flowchart TD
+    A[excalidraw repo] --> B[excalidraw-app]
+    A --> C[packages]
+    A --> D[dev-docs]
+    A --> E[examples]
+    A --> F[.github]
+    B --> B1[Runtime behavior]
+    C --> C1[Reusable logic]
+    D --> D1[Engineering docs]
+    E --> E1[Integration references]
+    F --> F1[CI and review flow]
+```
+
+## First 5 Working Days Plan
+
+### Day 1 - Setup and product context
+- Read `README.md` and contribution docs.
+- Run the app locally.
+- Identify where the app bootstraps and where state lives.
+
+### Day 2 - Trace one feature end-to-end
+- Choose one feature (e.g., export, frames, dark mode).
+- Follow UI interaction -> state change -> render/update.
+- Note key files and boundaries.
+
+### Day 3 - Package and docs deep-dive
+- Inspect related modules in `packages/`.
+- Compare behavior with docs in `dev-docs/`.
+- Read one merged PR to match team style.
+
+### Day 4 - First contribution
+- Submit a safe docs/small fix PR.
+- Run project checks locally.
+- Include clear validation steps in PR text.
+
+### Day 5 - Knowledge handoff
+- Share a concise onboarding report:
+  - explored modules,
+  - delivered change,
+  - open technical questions.
+
+## Development Workflow
+
+```mermaid
+sequenceDiagram
+    participant Dev as Developer
+    participant Br as Feature Branch
+    participant CI as CI Pipeline
+    participant Rev as Reviewer
+    Dev->>Br: Create branch and implement change
+    Dev->>Br: Commit and push
+    Br->>CI: Trigger checks
+    CI-->>Dev: Lint/test feedback
+    Dev->>Rev: Open pull request
+    Rev-->>Dev: Request updates / approve
+    Dev->>Br: Final updates then merge
+```
+
+## Team Quality Expectations
+
+- Keep PR scope focused.
+- Explain problem, approach, and validation clearly.
+- Add or update tests for behavior changes.
+- Surface uncertainty early during review.
+
+## Token Evaluation (Onboarding Scope)
+
+The estimates below help plan AI-assisted exploration sessions.
+
+Method:
+- Approximation: `tokens ~= characters / 4`
+- Usage: prompt/context planning, not billing-exact values
+
+| Project Area | Approx chars | Approx tokens | Onboarding value |
+|---|---:|---:|---|
+| Root `README.md` | 8,000 | 2,000 | Product and setup overview |
+| Contribution docs (`CONTRIBUTING`, intro docs) | 14,000 | 3,500 | Team workflow and standards |
+| `dev-docs/docs/codebase/*` | 25,000 | 6,250 | Internal architecture understanding |
+| `excalidraw-app/` selected feature files | 120,000 | 30,000 | Main runtime behavior |
+| `packages/` selected modules | 80,000 | 20,000 | Shared APIs and logic |
+| 3-5 merged PRs and discussions | 18,000 | 4,500 | Review style and expected quality |
+
+Estimated first-pass onboarding budget: **~66,250 tokens**.
+
+## Recommended AI Study Strategy
+
+- Start docs-first, then move to one feature slice.
+- Use narrow prompts (single question, single module).
+- Avoid full-repo ingestion in one context window.
+- Maintain onboarding notes as external memory across sessions.
+