CLI: Add `--quiet` flag to `dx build` and `dx serve` to reduce output verbosity

## Environment
- **dx version:** 0.7.0-rc.0 (a519fba)
- **rustc:** 1.89.0 (29483883e 2025-08-04)
- **OS:** macOS 14.5.0 (Darwin 24.5.0 arm64)

## Description
`dx build` currently outputs every single package compilation (e.g., "Compiled [1/411]: unicode_ident"), resulting in hundreds of log lines. There's `--verbose` for more output, but no `--quiet` for less.

## Current Behavior

Running `dx build --fullstack` on a typical project produces:
- 799 "Compiled [N/411]" progress lines
- 1218 total output lines
- Only way to reduce is redirecting stderr

Example output:
```
[ 1.605s] INFO Compiled [ 1/411]: unicode_ident
[ 1.605s] INFO Compiled [ 2/411]: build-script-build
[ 1.605s] INFO Compiled [ 3/411]: build-script-build
... (796 more lines) ...
```

## Requested Feature

Add `--quiet` flag (similar to `cargo build --quiet`) that:
- Suppresses per-package compilation progress
- Still shows warnings and errors
- Still shows final build result
- Works for `dx build` and `dx serve`

Example:
```bash
dx build --quiet
# Output:
# [ 32.974s] INFO Build completed successfully in 4262ms
```

## Use Cases
- **CI/CD pipelines:** Reduce log clutter, focus on warnings/errors
- **AI tools:** Smaller context window usage when monitoring builds
- **Local development:** Less terminal spam when you just want the result
- **Log aggregation:** Cleaner logs in production build systems