feat(source-control): recover conflict intent in the pull-request base-merge step and add a CI conflict mode #13137

Part of #7576 (PR 2 of 3).

Problem

/source-control:resolve-conflicts is to be removed, with its intent-reading step living in /source-control:pull-request's base-merge step and escalating only when the two intents cannot both hold (owner, interview Q541 and Q568). #7576's body says the base-merge step lands before the removal, so this PR moves the discipline and adds the CI mode; PR 3 removes the skill.

Done when

• plugins/source-control/skills/pull-request/reference/ready-for-review.md §2.5.2 ("Refresh the base with a merge") carries the resolve-conflicts discipline: map the stop, recover both sides' intent, compose by default, semantic-conflict sweep, verify (from plugins/source-control/skills/resolve-conflicts/SKILL.md Task steps 1-5 and its "Never pick a side" and "--abort is not a resolution strategy" rules), and escalates only when both intents cannot hold.
• The repository-specific version-bump and changelog-fragment rule (the "Plugin version-bump collisions" bullet of resolve-conflicts Task step 3, SKILL.md:85) lives in /source-control:pull-request, and plugins/source-control/skills/babysit-loop/SKILL.md:435, plugins/source-control/skills/babysit-prs/reference/loop.md:243-249 and plugins/source-control/skills/babysit-prs/reference/orchestration.md:582-588 cite its new home.
• /source-control:pull-request gains a CI conflict mode that does what resolve-conflicts' ## CI mode does today: it reads the runner's conflicts.json, runs no command, never edits a workflow file, and escalates unresolvable paths. Its argument form follows address-feedback's comments --ci --unattended (docs/conventions/pr-pipeline.yaml:95).
• plugins/source-control/skills/pull-request/reference/ready-for-review.md:125, plugins/source-control/skills/pull-request/reference/create.md:246,264 and plugins/source-control/skills/pull-request/scripts/push-branch.sh:481 point at the base-merge step instead of the resolve-conflicts skill.
• A changelog fragment for source-control.

Filed by Issue Triage from the #7576 spec (agent choice, verified against main). This was generated by AI.