Add Taskfile for unified dev workflow across all components (#6080)

## Add Taskfile for unified dev workflow

### Summary
- Introduces [Taskfile](https://taskfile.dev/) as the single CLI entry
point for all development workflows across backend, frontend, engine,
Docker, and desktop
- ~80 tasks organized into 6 namespaces: `backend:`, `frontend:`,
`engine:`, `docker:`, `desktop:`, plus root-level composites
- All CI workflows migrated to use Task
- Deletes `engine/Makefile` and `scripts/build-tauri-jlink.{sh,bat}` —
replaced by Task equivalents
- Removes redundant npm scripts (`dev`, `build`, `prep`, `lint`, `test`,
`typecheck:all`) from `package.json`
- Smart dependency caching: `sources`/`status`/`generates`
fingerprinting, CI-aware `npm ci` vs `npm install`, `run: once` for
parallel dep deduplication

### What this does NOT do
- Does not replace Gradle, npm, or Docker — Taskfile is a thin
orchestration wrapper
- Does not change application code or behavior

### Install
```
npm install -g @go-task/cli    # or: brew install go-task, winget install Task.Task
```

### Quick start
```
task --list       # discover all tasks
task install      # install all deps
task dev          # start backend + frontend
task dev:all      # also start AI engine
task test         # run all tests
task check        # quick quality gate (local dev)
task check:all    # full CI quality gate
```

### Test plan
- [ ] Install `task` CLI and run `task --list` — verify all tasks
display
- [ ] Run `task install` — verify frontend + engine deps install
- [ ] Run `task dev` — verify backend + frontend start, Ctrl+C exits
cleanly
- [ ] Run `task frontend:check` — verify typecheck + lint + test pass
- [ ] Run `task desktop:dev` — verify jlink builds are cached on second
run
- [ ] Verify CI passes on all workflows

---------

Co-authored-by: James Brunton <[email protected]>

CONTRIBUTING.md

@@ -15,6 +15,19 @@ Before you start working on an issue, please comment on (or create) the issue an
 Once you have been assigned an issue, you can start working on it. When you are ready to submit your changes, open a pull request.
 For a detailed pull request tutorial, see [this guide](https://www.digitalocean.com/community/tutorials/how-to-create-a-pull-request-on-github).
 
+## Development Quick Start
+
+This project uses [Task](https://taskfile.dev/) as a unified command runner. After cloning:
+
+1. Install the `task` CLI: https://taskfile.dev/installation/
+2. Run `task install` to install all dependencies
+3. Run `task dev` to start backend + frontend
+4. Run `task check` before submitting a PR
+
+Run `task --list` to see all available commands.
+
+## Pull Request Guidelines
+
 Please make sure your Pull Request adheres to the following guidelines:
 
 - Use the PR template provided.
@@ -39,9 +52,10 @@ If, at any point in time, you have a question, please feel free to ask in the sa
 
 ## Developer Documentation
 
-For technical guides, setup instructions, and development resources, please see our [Developer Documentation](devGuide/) which includes:
+For technical guides, setup instructions, and development resources:
 
-- [Developer Guide](devGuide/DeveloperGuide.md) - Main setup and architecture guide
+- [Developer Guide](DeveloperGuide.md) - Main setup and architecture guide
+- [Taskfile.yml](Taskfile.yml) - Unified task runner for all build/dev/test/lint commands
 - [Exception Handling Guide](devGuide/EXCEPTION_HANDLING_GUIDE.md) - Error handling patterns and i18n
 - [Translation Guide](devGuide/HowToAddNewLanguage.md) - Adding new languages
 - And more in the [devGuide folder](devGuide/)