feat: merge tool polish and :Differ continue

[undont]

Summary

• :Differ continue finishes the merge, rebase, cherry-pick, revert or am in progress from nvim: continue, continue and edit the message, skip (confirmed) or abort
• gC on the panel, diff view and merge result opens the same "continue" menu: skip or abort while conflicts are left (abort only for a merge), continue once every conflict is resolved
• git keeps the commit step: its editor opens the message in a buffer in this nvim, so hooks, cleanup, authorship and later rebase steps (reword, squash) behave as in a terminal
• :w and close commits the message; closing unwritten or emptying it cancels that commit and leaves the operation stopped
• continue and skip run in a floating terminal, allowing git to prompt there and a gpg passphrase can draw
• the merge tool no longer jumps on save: a write stages a marker-free result, ]f/[f save, stage and step between conflicted files, and the winbar shows file and conflict progress
• files changed on one side and deleted on the other open in the merge tool; the take keys keep or delete them
• binary conflicts now open in the merge tool; the take keys check out a side and stage it
• conflicts the merge tool still can't open (symlinks, file/directory) are skipped and named instead of stopping the flow
• the session follows git changes made outside it: the file count refreshes, and it closes when the operation moves on or the file is resolved elsewhere
• the local panel names the operation in progress ((merging) topic → main) and how to continue it
• a failed git add no longer reports the file staged
• inline diagnostic plugins no longer show LSP errors over the result's conflict markers
• a file reopened after the merge tool closes no longer keeps its winbar, sign column and scrollbind
• the merge tool hands back the file's own buffer-local keymaps on close
• the panel footer follows HEAD after a commit

Test plan

• make check clean (504 unit, 932 nvim, go)
• lots of manual testing with various scenarios