fix: avoid gitsync proposal ref conflicts

[chubes4]

Summary

• Derive keyed GitSync proposal branches as gitsync/<slug>-<proposal> so they do not nest under the sticky gitsync/<slug> ref.
• Preserve default submit behavior on gitsync/<slug>.
• Update CLI/ability help and smoke coverage for sticky/keyed branch coexistence.

Fixes #453.
Related: #451.

Verification

• php tests/smoke-gitsync.php
• php -l inc/GitSync/GitSyncProposer.php && php -l inc/Cli/Commands/GitSyncCommand.php && php -l inc/Abilities/GitSyncAbilities.php
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Implemented the branch derivation fix, updated docs/tests, and ran verification; Chris remains responsible for review and merge.

[chubes4]

Closing as duplicate of #454. #454 has the same branch-shape fix plus broader smoke coverage for coexistence with the legacy sticky ref.