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>

engine/AGENTS.md

@@ -4,9 +4,23 @@ This file is for AI agents working in `engine/`.
 
 The engine is a Python reasoning service for Stirling. It plans and interprets work, but it does not own durable state, and it does not execute Stirling PDF operations directly. Keep the service narrow: typed contracts in, typed contracts out, with AI only where it adds reasoning value.
 
+## Commands
+
+All engine commands can be run from the repository root using Task:
+
+- `task engine:check` — run all checks (typecheck + lint + format-check + test)
+- `task engine:fix` — auto-fix lint + formatting
+- `task engine:install` — install Python dependencies via uv
+- `task engine:dev` — start FastAPI with hot reload (localhost:5001)
+- `task engine:test` — run pytest
+- `task engine:lint` — run ruff linting
+- `task engine:typecheck` — run pyright
+- `task engine:format` — format code with ruff
+- `task engine:tool-models` — generate tool_models.py from frontend TypeScript defs
+
 ## Code Style
 
-- Keep `make check` passing.
+- Keep `task engine:check` passing.
 - Use modern Python when it improves clarity.
 - Prefer explicit names to cleverness.
 - Avoid nested functions and nested classes unless the language construct requires them.