feat(sources): single-issue source (favorites + open-issue-by-id)

I wired the issue kind into the same pipeline every other source uses, so a favorited issue renders in the buffer as its own subtree instead of the v1 browser punt. The new pearl-open-issue-by-id command does the same for an issue you type by identifier or id. It's handy when someone hands you ENG-123 and you'd rather read it in Pearl than a browser.

Linear's issue(id:) accepts both the UUID and the human identifier, so there's no resolution step. I fetch by whatever was given and build the :type issue source from the returned node, so the stored id and identifier stay authoritative. Refresh re-fetches the one issue and merges by LINEAR-ID like the filter and view sources.

Tests cover the source constructor, the header round-trip, and the fetch/render and command paths across Normal, Boundary, and Error.

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