Add Junie by JetBrains support to lemonade launch

[mashan555]

Spec Driven Development

This PR:

• fixes something and does not need a WG/RFC.
• is within the scope of WG
• has approved RFC RFC: support Junie by JetBrains in lemonade launch #3608

Summary

lemonade launch junie --model adds Junie as a launch agent, with --model, --api-key, and --agent-args behaving the same way as for the other agents. On launch, Lemonade loads the selected model, writes a Junie profile to ~/.junie/models/lemonade.json (or under $JUNIE_HOME) pointing to Lemonade's OpenAI-compatible endpoint and including the model's ctx_size, then starts Junie with that profile. Only this profile is updated, so other Junie profiles are left untouched, and if Junie is not installed the command prints installation instructions. Junie is also documented in the lemonade launch help output and integration guide.

Scope

• This PR addresses one clear issue or change.
• I reviewed the full diff myself before submitting.
• I removed unrelated local changes.
• I kept refactoring separate unless it is required for this change.

Testing

• The code change has been locally tested.

Testing details:

Six new CLI tests in test/server_cli2.py cover the launch arguments, generated profile, --api-key, $JUNIE_HOME, preservation of other Junie profiles, and the Windows shim name. They use a stub Junie binary, so no real installation or inference backend is needed.

cd test
../.venv/bin/python -m unittest server_cli2.PersistentServerCLIClientTests -k junie -v

Result on macOS: five passed, the Windows test was skipped.

Also verified with Junie 26.9.14 on macOS:

./build/lemonade load Qwen3.5-4B-GGUF
junie --model custom:lemonade

Starting Junie against the generated profile works: the profile is accepted and the prompt is answered by the local model. A check on a native Windows machine would still be useful.

Documentation

• Documentation is not affected by this change.
• Documentation is affected and has been updated.

Breaking Changes

• This PR introduces breaking changes.
• This PR does not introduce breaking changes.