Improve the CodeWhale sidebar experience with clearer agent status, panel navigation, and collapse controls. - #10

…anel overlay

The three sidebar tabs became peers in #10, which left
brotherwhale.showThreadList contributed, documented and read by the chat
provider while the webview only assigned it to a no-op: a user who set it
saw the Threads tab anyway, and nothing said so. Remove it end to end — the
contribution, its package.nls strings, the three reads, the no-op, the
export and the runtime message field the webview consumed.

Also:

- Esc collapses an open panel. The overlay covers the chat, so the ✕ was the
  only way out; an Esc that reaches an input (slash menu, task draft) is left
  to that input.
- renderSessions() removed .session-empty-msg while appending .work-empty, so
  an empty result list stacked another "no conversations" block on every
  render instead of replacing it. Both list renderers now remove what they
  create, keeping the hint and the search bar.
- The toolbar's thread-count id/class (and the debug probe key) is named
  agent-panel-toggle, which is what the element has been since #10.
- The zh Activity hint no longer mixes "Agent" into Chinese.

README and README_zh now describe the Sessions / Threads / Activity tabs, the
threads panel as an overlay over the chat, and how it closes; the settings
table drops the removed showThreadList row, and the contributor list credits
#10 to @eoli.

AGENTS.md records the new sidebar structure, and CHANGELOG gets an Unreleased
section covering #10 and these follow-up fixes.

#10 made the panel an absolutely positioned overlay — opaque background, drop
shadow, z-index 20 — so that opening it would not take width from the chat.
Taking that width was the point: the overlay covered the messages the panel
exists to navigate, so the two could never be read together, and the resize
grip (also absolute, with its `left` written on every drag and on load) could
only ever widen a panel that was hiding content.

The panel is an in-flow flex child of #layout again, so the chat area is
squeezed rather than covered, and the grip stands in its own place in the row:
no left offset is written during a drag, and the saved width applies to the
panel alone. The three tabs, the ✕, the Esc handler, the attention badge and
the width persistence are untouched.

Measured in headless Chromium against the generated webview HTML at 900x700 —
overlay: panel 0–220 on top of a chat that still spans 0–900 (220px covered);
restored: panel 0–220, grip 220–224, chat 224–900, nothing covered, and the
chat width is the layout width minus panel and grip.

Also:

- The CSS and Esc-handler comments no longer explain the layout as an overlay.
- README / README_zh say the panel sits beside the chat and takes its width
  from it, and the #10 credit no longer claims the overlay.
- AGENTS.md's 侧边栏状态 records the in-flow panel, so it does not drift back
  to absolute positioning.
- CHANGELOG drops the overlay sentence from the Unreleased sidebar entry (it
  never shipped) and records the regression under Bug Fixes.
- vitest 946/946, tsc --noEmit, eslint src and the webpack build all pass.