fix(explore): keep the event-pane chevron live on skill pages

[jrosskopf]

Summary

Once the left event pane was collapsed it could not be re-opened on a skill page — the expand chevron was dead. (Instance pages toggled fine, so it looked intermittent.)

Cause. The event pane auto-minimizes on skill pages (skills carry no events), modelled as a derived OR:

effective = leftCollapsed || isSkill   // chevron writes leftCollapsed

On a skill, isSkill is true, so the OR is stuck true regardless of the chevron — clicking expand set leftCollapsed = false, but false || true == true.

Fix. Make leftCollapsedProvider an explicit choice (bool?, null = follow the page-type default) that effective prefers over the skill default, and reset it to null in _SplitBody when the focused page flips skill↔instance. The chevron is now always authoritative for the current page (it can re-open the event pane on a skill), while skills still auto-minimize by default and instances still show events.

Verified live via chrome-devtools on the seeded :8080 gateway: on the customer skill page the auto-minimized event pane now re-opens when the chevron is clicked.

Test plan

• test/crm/crm_workspace_test.dart → new the event-pane chevron re-opens an auto-minimized skill page: drives the real chevron on a skill page and asserts the pane expands. Verified red under the old OR-logic, green after the fix.
• test/crm/left_pane_minimize_test.dart: updated for the explicit-choice semantics — skill auto-minimizes by default, an explicit expand wins on a skill, instances default to shown and honour an explicit collapse.
• test/crm/crm_workspace_test.dart collapsing the left view… still green (override => true is a valid bool?).
• flutter analyze clean; full suite green (94 tests).
• docs/notes/discovered/2026-05-31-dead-event-pane-chevron-on-skill.md documents the anti-pattern (user flag OR-ed with a derived condition → control inert where the condition holds).

🤖 Generated with Claude Code