feat(cli): link release notes after hunk update

[shashank-100]

Closes #951.

hunk update confirms the version it installed but never says what changed. The changelog already publishes a stable anchor per release, so the notes are addressable.

Updated hunk to 0.21.0.
Release notes: https://hunk.dev/changelog/0.21/#v0-21-0

Printed after an update, when already current, and for --check.

• Plain URL, not OSC-8 — this output gets piped and captured in CI.
• No link for a version with no changelog page (dev builds, 0.0.0-unknown) rather than a URL to a 404.
• --check links the latest available version, which is what the surrounding lines describe.

The URL rules live in core/run/releaseNotes.ts rather than importing from scripts/, since that is build tooling the CLI cannot depend on. A test reads every generated page under website/src/content/docs/changelog/ and asserts the builder reproduces each published URL, so the two cannot drift.

Verification

bun run typecheck, bun run lint, bun run deps:check clean. bun run test: 3927 pass, 2 fail — both pre-existing, reproduced on a clean tree at this base (install VM contract hits a macOS /var -> /private/var symlink mismatch; Jujutsu source reading passes when not run under full-suite load). This diff touches only selfUpdate and the new releaseNotes module.

Checked the generated URLs against the live site — all 200 with the anchor present in the served HTML:

0.21.1         https://hunk.dev/changelog/0.21/#v0-21-1
0.21.0-beta.0  https://hunk.dev/changelog/0.21/#v0-21-0-beta-0
0.18.2         https://hunk.dev/changelog/0.18/#v0-18-2
0.13.0         https://hunk.dev/changelog/0.13/#v0-13-0

And on the compiled binary:

$ hunk update --check
hunk 0.21.1 (installed with npm)
latest 0.21.1
Hunk is up to date.
Release notes: https://hunk.dev/changelog/0.21/#v0-21-1

Verified on macOS.

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