fix(update): drop install-v1 Accept header on dist-tag fetch

[scottlovegrove]

Summary

fetchLatestVersion was sending Accept: application/vnd.npm.install-v1+json on the dist-tag endpoint (/<package>/<tag>). The npm registry rejects that combination with HTTP 406 — the abbreviated metadata format only applies to the package-doc endpoint (/<package>), not dist-tag resolutions.

Reproduced live:

$ curl -sS -o /dev/null -w "%{http_code}\n" https://registry.npmjs.org/@doist/todoist-cli/latest
200
$ curl -sS -o /dev/null -w "%{http_code}\n" -H "Accept: application/vnd.npm.install-v1+json" https://registry.npmjs.org/@doist/todoist-cli/latest
406

In production this surfaced as:

$ td update
✖ Checking for updates...
Error: UPDATE_CHECK_FAILED
Failed to check for updates: Registry request failed (HTTP 406)

The dist-tag endpoint already returns a small single-version document, so the abbreviated-format header was never buying anything anyway — drop it.

Test plan

• npm test — 184 tests pass; existing assertion updated; new regression test asserts fetch is called without an options argument so the bad header can't reappear silently
• npm run check / type-check / build clean
• Smoke against the real registry: fetchLatestVersion({ packageName: '@doist/todoist-cli', channel: 'stable' }) returns 1.62.1
• Smoke from todoist-cli (tarball install): td update --check, td update --channel, td update --check --json all succeed against the real npm registry

🤖 Generated with Claude Code

[doistbot]

This PR resolves an update check failure by removing an incompatible Accept header from npm registry dist-tag requests. Catching the HTTP 406 error and adding a regression test ensures the update mechanism remains reliable in production. There is a minor opportunity to simplify the new regression test by leveraging vi.mocked(fetch) and relying on toHaveBeenCalledWith for exact argument matching rather than manually inspecting the call array.

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[doist-release-bot]

🎉 This PR is included in version 0.7.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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