Add Copilot development guide for repository onboarding

.github/copilot-instructions.md

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+# Quartz Syncer - Copilot Development Guide
+
+## Project Overview
+
+Obsidian plugin for publishing notes to Quartz static-site generator. TypeScript/Svelte project that compiles to `main.js` bundle.
+
+**Tech Stack**: TypeScript, Svelte, esbuild | **Node**: 20.6.0 (`.nvmrc`) | **Entry**: `main.ts` (534 lines)
+
+## Build Commands
+
+**CRITICAL: Always run `npm install` first** after cloning or when dependencies change.
+
+### Essential Commands
+```bash
+npm install              # Install deps (~50s, shows 3 known svelte warnings - ignore)
+npm run build            # Production build (~800ms, creates 1.0MB main.js) - CI uses this
+npm run dev              # Dev build (3.3MB with sourcemaps, copies to docs/.obsidian/)
+npm run test             # Jest tests (~6s, 2 suites, 7 tests)
+npm run lint             # ESLint check (~3s)
+npm run format           # Auto-fix formatting with Prettier
+npm run check-formatting # Check Prettier formatting
+npm run typecheck        # TypeScript type check (~3s)
+```
+
+### Pre-Commit Validation (matches CI)
+```bash
+npm run lint && npm run check-formatting && npm run typecheck && npm run build && npm run test
+```
+
+## CI/CD (GitHub Actions)
+
+**Runs on**: Every PR and push to main/master. **All 4 jobs must pass**:
+1. `lint-and-check-formatting`: ESLint + Prettier
+2. `build`: Production build (uploads main.js artifact)
+3. `run-tests`: Jest tests  
+4. `run-typecheck`: TypeScript check
+
+**Also runs**: CodeQL security scanning (weekly + on PRs), Release workflow (on tags)
+
+**Fix CI failures**:
+- Lint: `npm run format && npm run lint-fix`
+- Types: Fix errors from `npm run typecheck`
+- Tests: Fix failures from `npm run test`
+
+## Project Structure
+
+### Key Source Directories
+```
+src/
+├── compiler/          # Frontmatter/plugin compilation (Dataview, Excalidraw, Mermaid)
+├── models/            # Data models, settings.ts (plugin settings interface)
+├── publishFile/       # File validation, metadata, PublishFile.ts
+├── publisher/         # Publishing workflow, Publisher.ts, *.test.ts
+├── repositoryConnection/ # GitHub API (RepositoryConnection.ts, QuartzSyncerSiteManager.ts)
+├── ui/                # Svelte components (Icon.svelte, LineDiff.svelte, TreeView/)
+├── utils/             # Utilities (utils.ts, utils.test.ts, regexes.ts)
+└── views/             # Settings UI (QuartzSyncerSettingTab.ts, PublicationCenter/, SettingsView/)
+```
+
+### Root Files
+- `main.ts` - Plugin entry point (DEFAULT_SETTINGS, main plugin class)
+- `package.json` - npm scripts (dev, build, test, lint, format, etc.)
+- `tsconfig.json` - TypeScript config (extends @tsconfig/svelte, target es2024, commonjs)
+- `esbuild.config.mjs` - Build config (bundles to main.js, external: obsidian/electron)
+- `eslint.config.mjs` - ESLint flat config (TS/Svelte, ignores: *.js, scripts, docs, content)
+- `.prettierrc.json` - Prettier config (tabs, 80 chars, semicolons, double quotes)
+- `manifest.json` - Obsidian plugin manifest (version, author, minAppVersion)
+- `justfile` - Optional task runner (check, full, dev, prod commands)
+
+## Configuration & Code Style
+
+**ESLint** (`eslint.config.mjs` - flat config):
+- Ignores: `**/*.js`, scripts, test vaults, content, docs
+- Rules: No `any` type, unused vars prefixed with `_`, prettier/prettier errors, blank lines before returns/ifs
+- Svelte files must use `<script lang="ts">`
+
+**Prettier** (`.prettierrc.json`):
+- Tabs (not spaces), 80 chars, semicolons required, double quotes
+
+**TypeScript** (`tsconfig.json`):
+- Target es2024, commonjs, `noImplicitAny: true`
+
+**Build** (`esbuild.config.mjs`):
+- Entry: `main.ts` → `main.js` (CommonJS)
+- External: obsidian, electron, Node builtins
+- Svelte CSS injected into bundle
+
+**Git**: `main.js` gitignored (only in releases), `node_modules`, test vault workspace files
+
+## Known Issues & Workarounds
+
+**Security warnings**: 3 moderate svelte vulnerabilities (from obsidian-dataview dependency) - **safe to ignore**, don't run `npm audit fix --force`
+
+**Build size warning**: esbuild shows ⚠️ for main.js size (1.0MB prod, 3.3MB dev) - **expected and normal**
+
+**MODULE_NOT_FOUND after install**: If dependencies fail after `npm install`, run:
+```bash
+rm -rf node_modules package-lock.json && npm install
+```
+
+## Development Tips
+
+1. **Fast build** (~800ms) - rebuild frequently to catch errors early
+2. **Use tabs, not spaces** - enforced by Prettier
+3. **No `any` types** - enforced by ESLint, all params need explicit types  
+4. **Test files**: `*.test.ts` suffix, placed next to code, uses Jest with ts-jest
+5. **Svelte components**: Must have `<script lang="ts">`, CSS injected into bundle
+6. **Main entry**: `main.ts` has plugin lifecycle (onload, onunload, settings)
+7. **GitHub API**: `src/repositoryConnection/` for API communication
+8. **Compilation logic**: `src/compiler/` converts Obsidian → Quartz format
+9. **Version updates**: Use `version-bump.mjs` to sync package.json, manifest.json, versions.json
+10. **Trust these instructions first** - only explore codebase if info here is incomplete/incorrect