fix: accept exact base branch refs in Herdr picker

[leesangb]

Summary

• preserve unmatched fzf queries for the Herdr base-branch picker
• accept the query only when it exactly matches an existing Git ref
• keep invalid refs and Escape as cancellation paths
• add regression coverage, documentation, and a patch changeset

Verification

• bun test
• bun run typecheck
• bun run build:herdr-plugin
• git diff --check