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 <jbrunton96@gmail.com>

Taskfile.yml

@@ -0,0 +1,128 @@
+version: '3'
+
+output: prefixed
+
+includes:
+  backend:
+    taskfile: .taskfiles/backend.yml
+    dir: .
+  frontend:
+    taskfile: .taskfiles/frontend.yml
+    dir: frontend
+  engine:
+    taskfile: .taskfiles/engine.yml
+    dir: engine
+  docker:
+    taskfile: .taskfiles/docker.yml
+    dir: .
+  desktop:
+    taskfile: .taskfiles/desktop.yml
+    dir: frontend
+
+tasks:
+  # ============================================================
+  # Setup & Prerequisites
+  # ============================================================
+
+  install:
+    desc: "Install all project dependencies"
+    cmds:
+      - task: frontend:install
+      - task: engine:install
+
+  # ============================================================
+  # Development
+  # ============================================================
+
+  dev:
+    desc: "Start backend + frontend concurrently"
+    deps:
+      - backend:dev
+      - frontend:dev
+
+  dev:all:
+    desc: "Start backend + frontend + engine concurrently"
+    deps:
+      - backend:dev
+      - frontend:dev
+      - engine:dev
+
+  # ============================================================
+  # Build
+  # ============================================================
+
+  build:
+    desc: "Build all components"
+    cmds:
+      - task: backend:build
+      - task: frontend:build
+
+  # ============================================================
+  # Test
+  # ============================================================
+
+  test:
+    desc: "Run ALL tests (backend + frontend + engine)"
+    cmds:
+      - task: backend:test
+      - task: frontend:test
+      - task: engine:test
+
+  # ============================================================
+  # Lint & Format
+  # ============================================================
+
+  lint:
+    desc: "Run all linters"
+    cmds:
+      - task: frontend:lint
+      - task: engine:lint
+
+  fix:
+    desc: "Auto-fix all components"
+    cmds:
+      - task: backend:fix
+      - task: frontend:fix
+      - task: engine:fix
+
+  format:
+    desc: "Auto-fix formatting across all components"
+    cmds:
+      - task: backend:format
+      - task: frontend:format
+      - task: engine:format
+
+  format:check:
+    desc: "Check formatting across all components"
+    cmds:
+      - task: backend:format:check
+      - task: frontend:format:check
+      - task: engine:format:check
+
+  # ============================================================
+  # Quality Gate
+  # ============================================================
+
+  check:
+    desc: "Quick quality gate for local development"
+    cmds:
+      - task: backend:check
+      - task: frontend:check
+      - task: engine:check
+
+  check:all:
+    desc: "Full CI quality gate"
+    cmds:
+      - task: backend:check
+      - task: frontend:check:all
+      - task: engine:check
+
+  # ============================================================
+  # Clean
+  # ============================================================
+
+  clean:
+    desc: "Clean all build artifacts"
+    cmds:
+      - task: backend:clean
+      - task: engine:clean