#28184 carry of #28094: retain cmux-tui v2 key compatibility

Carry of #28094: ci(cmux-tui): skip docs-only artifact publishes.

Fixes the BLOCK finding at 4f23eb8: docs-only v2 keys now use the new docs-excluding definition, while v2-legacy retains the pre-change algorithm and is used as the compatibility lookup for immutable artifacts already published under the old v2 key. The tree-key and publication tests pass.

Original PR: https://github.com/manaflow-ai/cmux/pull/28094

Agent session: agent session

---

^{Need help on this PR? Tag @codesmith-bot with what you need.}

---

Summary by cubic

Carries #28094's docs-only artifact skip and keeps already-published v2 artifacts reachable.

• GitHub now filters pushes that change only cmux-tui/docs/**, so the artifact publisher no longer runs for docs-only commits.
• The v2 tree key omits the cmux-tui/docs subtree, so a docs-only commit keeps the same published runtime key.
• A new v2-legacy key version preserves the original v2 algorithm and is used to look up artifacts already published under the old key.