document base-branch conflict resolution in issue-implement (#481) (#485)
Add a 'Resolving conflicts with the base branch' section to issue-implement making explicit that, when develop advances under an in-flight branch, rebasing onto the base and force-pushing your own PR with --force-with-lease is routine and pre-authorized — not a violation of the general 'never force-push without approval' rule. Add a matching pointer in the pr-watch USER half, where the post-PR conflict actually surfaces. Docs-only; addresses an agent stalling on a routine rebase. Ref #481.