feat(web): collapse same-worktree chats into one sidebar row

[enisze]

What

Chats spawned into the same git worktree via the in-chat worktree tab strip no longer each get their own sidebar row. A worktree now appears as a single sidebar row — the earliest-created chat — and its sibling chats are reachable only through the tabs inside that chat. Chats with no worktree are unchanged (one row each).

Applies to both sidebars: v1 (Sidebar.tsx) and v2 (SidebarV2.tsx, default on dev/nightly).

How

• New pure helper collapseWorktreeSiblings(threads, keyOf) in Sidebar.logic.ts — groups by environmentId + worktreePath, keeps the earliest-created chat per group (ties broken by id), preserves order, and returns a representativeKeyByThreadKey map.
• v1: collapses each project's thread list; the project status dot still reads all non-archived threads so a busy sibling isn't hidden. Mirrored in the jump-label/prewarm pass. Active-route highlight, collapsed-pin, and keyboard traversal remap to the representative row.
• v2: collapses before the active/snoozed/settled partitioning; remaps highlight, the "keep the open thread visible" shelf pull-ins, and keyboard traversal.
• 5 new unit tests for the helper.

Verification

• pnpm typecheck (apps/web): clean
• vp lint on changed files: clean
• Sidebar.logic.test.ts: 86/86 pass

Note on the representative

The representative is the earliest chat (stable title/position). In v2 the row's active/settled bucket follows that earliest chat's status — a known tradeoff of choosing stability over "surface the most recent activity."

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