Report whether an issue fork exists and stop setup-remote when it does not

[mglaman]

Fixes #379.

issue:get-fork printed fork URLs for a fork that did not exist, and --format=json and --format=llm gave no way to tell that from a fork with no branches. issue:setup-remote then added the remote, failed on the fetch, and left the dead remote behind.

What changed

• IssueForkResult gains exists, set from the outcome of the project lookup the action already made. No extra request. The branch lookup runs separately, so a failed branch listing cannot report a missing fork.
• All formats print it: "exists": false in json, <exists>false</exists> in llm, **Exists:** no in md. The text format now distinguishes "Fork does not exist yet" from "Fork exists but has no branches yet".
• issue:setup-remote throws before adding the remote when the fork does not exist. issue:checkout stops with the same message instead of offering to run setup-remote.
• The bundled drupalorg-cli and drupalorg-work-on-issue skills document the flag and the new error.

Testing

Unit tests cover a fork with branches, a fork with no branches, a 503 on the branch listing, a missing fork, and that setup-remote never reaches git when the fork is missing.

Live, without a token, against poll issue 3007808 which has no fork:

drupalorg issue:get-fork 3007808 --format=llm --no-cache   # <exists>false</exists>
drupalorg issue:setup-remote poll#3007808                   # stops, no remote added
drupalorg issue:checkout poll#3007808 -n                    # stops, exit 1

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