v0.20.0

loaders: staged setup + staged indexing (CodeGraph-style) (#465)

`sem setup` used to print a flat list of `✓ Created …` lines. Now it
runs as a small staged tree with live per-step spinners, in the spirit
of CodeGraph's `init` loader:

```
  ⊕ setting up sem

  ◆ git diff → sem diff  aliased globally
  ◆ Claude Code hooks    already installed
  · pre-commit hook      not in a git repo — skipped

  ✓ sem is wired in — restart your Claude Code session to load it
     · warm graph + prompt-time context · entity-level git diff · blast-radius pre-commit
     revert anytime:  sem unsetup
```

Each step shows a braille spinner while it runs, resolving to a green
`◆` (did something), dim `·` (nothing to do / not applicable), or yellow
`⚠` (left a file untouched on purpose — e.g. an unparseable
settings.json). The two sub-installers now return a rendered `Outcome`
instead of printing, so the loader owns all output. Same idempotent
behaviour, uses the existing `indicatif` dep. Tests green.