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]>

docker/README.md

@@ -2,6 +2,23 @@
 
 This directory contains the organized Docker configurations for the split frontend/backend architecture.
 
+## Using Taskfile (Recommended)
+
+All Docker commands can be run from the project root using [Task](https://taskfile.dev/):
+
+```bash
+task docker:build            # Build standard image
+task docker:build:fat        # Build fat image (all features)
+task docker:build:ultra-lite # Build ultra-lite image
+task docker:build:frontend   # Build frontend-only image
+task docker:build:engine     # Build engine image
+task docker:up               # Start standard compose stack
+task docker:up:fat           # Start fat compose stack
+task docker:up:ultra-lite    # Start ultra-lite compose stack
+task docker:down             # Stop all running stacks
+task docker:logs             # Tail compose logs
+```
+
 ## Directory Structure
 
 ```