Deprecate the ignored traversal flag

[SihanTeng]

Summary

• Keep --traversal accepted so existing scripts continue to run.
• Mark it deprecated and explain that the crawl is always breadth-first.
• Document the compatibility period and add a regression test for --traversal dfs parsing.

Why

This is the flag cleanup split out of #76 as requested in review. The flag was never read, but deleting it immediately would turn previously successful commands into unknown flag failures.

Test plan

• go test -count=1 ./cli/ ./clone/ -run TestTraversalFlagRemainsAcceptedButDeprecated

[tamnd]

Deprecating rather than removing was the right call, this keeps existing scripts working. I squashed onto main and resolved the cli.md conflict with #76 by keeping its --exclude wording alongside your --traversal row. Worth knowing that MarkDeprecated also hides the flag from clone --help, which I checked on the merged tree and is what we want here since the docs table still lists it.