Fix compass pr create/merge task misattribution on shared branches

Multiple tasks in the same story can legitimately share a #+branch: value (e.g. every apply_safe_drift_* follow-on task in the "Refactor ores.codegen C++ generation" story is pre-authored with #+branch: feature/refactor-codegen-cpp, since they're expected to be picked up sequentially on one long-lived branch). compass pr create and compass pr merge both resolve "the task for this branch" by matching on #+branch: alone, with no tie-break when several tasks match. On PR #1873 this picked a different, unrelated, still-BACKLOG task twice: pr create recorded the PR against task_apply_safe_drift_analytics.org instead of the actual task_apply_safe_drift.org (silently editing the wrong file's #+pr: and PR table), and pr merge's guard then refused to merge, citing task_apply_safe_drift_compute.org (a third, also-unrelated task) being BACKLOG with an empty * Result. Both had to be manually diagnosed and corrected mid-PR.

Fix direction: disambiguate by picking the task whose state is STARTED (or most recently touched) among branch matches, rather than an arbitrary/first match; or warn and require an explicit --task argument when more than one task matches the current branch.

Silent misattribution is worse than a hard failure — it edits the wrong agile doc without any error, and the merge guard's false-positive can block (or, worse, get force-merged past) real state on an unrelated task. Found and worked around twice in one PR (#1873, "Apply safe drift to refdata-cpp (pilot)"); will very likely recur on every other apply_safe_drift_* task in this story unless fixed, since they all share the same branch by design.

• PR #1873 — both incidents happened here.
• doc/agile/versions/v0/sprint_25/refactor_codegen_cpp/task_apply_safe_drift.org — the correct task.
• doc/agile/versions/v0/sprint_25/refactor_codegen_cpp/task_apply_safe_drift_analytics.org — wrongly recorded by pr create.
• doc/agile/versions/v0/sprint_25/refactor_codegen_cpp/task_apply_safe_drift_compute.org — wrongly flagged by pr merge's guard.