feat(style): hyperlink PR numbers in submit output (#105)

[mvanhorn]

Closes #105.

What changed

`submit` now wraps the `PR #N` token in its progress line with an OSC 8 terminal hyperlink, so iTerm2 / kitty / recent macOS Terminal / GNOME Terminal users can click the PR number to jump to the URL. Terminals without hyperlink support (or sessions where output is piped) get a plain `text (URL)` fallback so the URL is still visible.

Before:

```
Updating PR #3150 for boneskull/additional-locations (base: master)... ok
```

After (TTY with OSC 8): the `PR #3150` is a clickable link to the PR URL.

After (no TTY / NO_COLOR):

```
Updating PR #3150 (https://github.com/boneskull/gh-stack/pull/3150) for boneskull/additional-locations (base: master)... ok
```

How

• New `internal/style/style.go` method `Hyperlink(text, url)`:
  • Colors/TTY enabled → `\x1b]8;;URL\x1b\TEXT\x1b]8;;\x1b\` (OSC 8).
  • Disabled → `"text (url)"`.
  • Empty URL → returns text alone, no parens.
• `cmd/submit.go:465` updated to use `s.Hyperlink(fmt.Sprintf("PR #%d", d.prNum), ghClient.PRURL(d.prNum))` in the `Updating ...` line. The `(base: parent)` suffix and `...` / `ok` / `failed` trailers are unchanged so existing test snapshots and TTY-detection logic are unaffected.
• `internal/style/style_test.go` (new) covers all three branches.

Why this matches the issue

Quoted from the issue (#105):

wherever terminal links are available, the text `PR #nnnn` should be the linked text.

That's the OSC 8 path. The fallback is what the issue showed as the alternate render.

Verification

• `go build ./...` clean.
• `go test ./internal/style/...` — 3 new tests pass.
• `go test ./cmd/...` — `cmd` tests pass (cached, no relevant impl changes there).
• `go test ./e2e/...` — same set of e2e dry-run tests fail on `main` as on this branch (pre-existing `git push -u origin main` env issue, unrelated to this change). Verified by stashing and re-running.