fix(common): use product-specific commit attribution

[c8dhjp4tyv-bit]

Recreated on the rewritten main after #1101 was auto-closed during repository maintenance. This carries the same reviewed change set on the new history.

Summary

• derive git commit attribution from the compile-time FREEBUFF_MODE flag
• keep Codebuff's existing footer for standard builds
• use Freebuff branding and domain for Freebuff binaries
• add regression coverage for both product variants

Fixes #997

Validation

Prior validation before the history rewrite:

• Freebuff build-flag attribution smoke check: passed
• targeted common tool tests: 10 passed
• common typecheck: passed
• git diff --check: passed
• Prettier check for changed files: passed
• common full test command: 1,158 passed; 37 existing checkout/snapshot failures remain in unrelated web/agent sources and one existing Zod schema expectation
• agent-runtime typecheck: existing missing agents-graveyard/researcher/researcher imports; no errors from changed files