Experimental: refine sidebar thread hierarchy

[brsbl]

Human comments

What was wrong

Thread grouping, section actions, menus, and visual hierarchy were being iterated in the same layers as approved sidebar controls and command-palette work.

What changed

• Isolate the experimental thread-list hierarchy, organization modes, filter/sort treatment, row actions, and section movement behavior.
• Keep non-sidebar plugin, Customize, terminology, and command-palette changes out of this layer.
• Build only on the experimental lifecycle foundation, not on the approved stack.

No public or wire contracts changed.

Screenshots

Pending; this PR is intentionally experimental and not merge-ready.

How you verified

• git diff --check
• Remote CI currently fails; this draft is not merge-ready.
• Exact branch web-app verification pending.
• Per repository policy, no CI-equivalent tests, typechecks, lint, or builds were run locally.

Fixes #

BB-Thread-ID: thr_ccffp4w2p2

AGENT GENERATED

[brsbl]

Closing this experimental layer for now. The approved sidebar and command-palette stack remains separate; any future thread-list work will restart incrementally from the stable baseline. Branch retained for reference.