Restore thread panel per channel across switches - #5593

What

Leaving a channel and returning now restores the thread panel the way it was left — open on the same thread, or closed. Previously every channel switch closed the panel: sidebar navigation builds a fresh /channels/$channelId URL with no thread search param, so the URL-backed panel state was lost on every forward navigation (back/forward already restored it per history entry).

How

Panel state stays URL-backed; the change adds a session-scoped per-channel memory that seeds the URL goChannel builds. Four small pieces, no relay/schema changes:

• channelPanelMemory.ts (new): tri-state per-channel memory — thread head id ("open on this thread"), null ("user left it closed", so a closed panel stays closed on return), no entry ("never visited"). Backed by sessionStorage following the thread-panel width precedent (useThreadPanelWidth): survives a reload, dies with the app session.
• useChannelThreadMemory.ts (new): continuously mirrors the raw ?thread value into the memory from ChannelScreen (one call line — the file sits near the 1000-line ceiling). Skips huddle transcripts (useHuddleThreadIsolation force-closes threads there) and the forum-post view. Continuous recording makes every exit path correct for free, and turns a thread snap-closed by useThreadTargetSync (deleted/unfetchable head) into memory = null — forgotten, never retried, so no restore loop is possible.
• goChannel: navigations that carry no explicit target (thread / messageId / agentSession / autoSend) seed thread from memory when building the search params. One choke point covers sidebar, tray, notifications, and DM opens; the URL is right the first time, so a switch stays exactly one history entry, and explicit targets plus back/forward win by construction. A restored thread whose head paged out of the initial window rides the existing deep-link splice (targetThreadRootId ← search.thread in channels.$channelId.tsx → ChannelRouteScreen fetch).
• resetChannelPanelMemory() wired into resetCommunityState() and added to the AGENTS.md singleton list — channel ids are community-local, so remembered panel state must not leak across a community switch (memory + its sessionStorage key are both cleared).

Not restored (out of scope): in-panel expanded replies and scroll position (plain React state); profile/agent-session panels (the memory value can grow to a partial record of those keys later without moving any wiring).

Testing

• channelPanelMemory.test.mjs — tri-state round-trip, per-channel isolation, sessionStorage write-through, redundant-write skip, reset semantics, storage-failure resilience (7 tests).
• tests/e2e/thread-panel-persistence.spec.ts (registered in the smoke project) — open thread in #general → switch to #random (panel closed there) → return (panel restored, same thread, URL carries thread=) → close panel → leave and return (stays closed).
• Full desktop unit suite: 4677/4677 pass. Neighboring e2e specs (navigation, channels, messaging, thread-focus-mode, threadpane-ultrawide, thread-unread) pass.
• biome check, tsc --noEmit, and check-file-sizes clean.