fix: use canonical www.codebuff.com URL for fallbacks - #840

Good catch and a minimal, focused diff. Using https://codebuff.com as a fallback when it 301-redirects to www.codebuff.com is a real footgun for fetch, which drops Authorization headers on cross-origin redirects per the Fetch spec's redirect handling for sensitive headers. Switching the fallback to the canonical www.codebuff.com in cli/src/hooks/use-freebuff-session.ts and cli/src/utils/error-handling.ts avoids the redirect entirely, and the doc-comment update in sdk/src/validate-agents.ts keeps the example consistent.

A few things worth checking before porting:

• Confirm https://www.codebuff.com is actually the domain that serves /api/v1/... — if the API is hosted only on the bare domain and www is just a marketing redirect, this could break things instead of fixing them. Worth a quick curl/test against both endpoints.
• No test coverage is added, though given the size and nature (a literal fallback string swap) that's understandable and probably not blocking for a maintainer to port by hand.
• It would help to also check if there are other places in the codebase using the same codebuff.com fallback pattern (e.g. web/ or shared config) that weren't touched here, even though those are out of scope for this mirror — a maintainer porting this should search for consistency.

Overall this is small, well-reasoned, and easy to verify/port by hand.