#3113 Docs: channel-aware extension push prompts and promote with a manifest (follow-up to #2939)

What changed

swamp-club#2939 (closing #2751 and #2753) changes swamp extension push and swamp extension promote. Take exact strings from the merged swamp PR; the summary below is the approved design.

• The final push confirmation names the channel, including the default: Push @x/y@V to registry (channel: stable)? [y/N], with what y and N each do.
• When the version already exists on a lower channel than requested, interactive push offers Promote (move the published build up; local files not uploaded), Bump (publish local files as a new version) or Stop, with no default. Same channel keeps today's error and bump prompt; a higher channel offers bump or stop only.
• The bump prompt says where the version already is and states what y and N do (N exits; nothing is pushed).
• Non-interactive runs (--json, --yes, --force) keep their behaviour and exit codes; --yes never promotes. In the cross-channel case the error names the channel and gives the exact promote command.
• Dry run: the version-exists registry check names both channels; its JSON row gains existingChannel and requestedChannel.
• swamp extension promote accepts a manifest path or extension directory: it reads name and version, reports the channel found, and exits 1 with 'Nothing to promote' when there is nothing to move.

Pages

• content/manual/reference/extensions/publishing.md: push flag table, dry-run Registry checks block, the version-exists row and its JSON fields, the promote section (arguments, examples).
• content/manual/how-to/extensions/create-and-publish.md: the publish and bump prompts.

Builds on #3071, which documents the version-exists check as it reads today. Do this once the #2939 swamp PR has merged.