feat(canvas): split-panel conversation canvas with cross-chat sibling awareness

[Legacynnn]

Summary

Splits the workspace center column into a recursive split-canvas of conversation panes, and makes agents aware of the other panes they share a canvas with.

• Split panes — Drag a conversation onto another's edge to split the center column into side-by-side (row) or stacked (col) panes, up to 4. Resize the splits, rearrange panes by drag, and close them. The pane tree is a pure, fully unit-tested model (src/shell/canvas/tree-model.ts); useCanvasState layers persistence + reducers on top.
• Persistent layout — The split is persisted per workspace to localStorage and survives navigating to other (non-split) sessions and back. It only dissolves when closed down to a single pane. Single sessions still render through the normal conversation path.
• ⌘W closes the focused pane — wired through the shell event bus (canvas-pane-closeable-changed / close-focused-canvas-pane) and global shortcut handlers.
• Cross-chat sibling awareness — When a send originates from a split, the frontend passes the sibling session IDs to the backend. The Helmor system prompt gains a "sibling panes" addendum listing each open sibling (title + session id) and the concrete helmor CLI commands to inspect, read, or hand them work (session list, session get-messages, send). The send command's blocking contract is made explicit so the agent doesn't poll. Single-pane sends are byte-for-byte unchanged (empty siblings ⇒ no addendum).

Test Plan

• New unit tests for the pane-tree model, persistence round-trip, DnD, and the useCanvasState hook (src/shell/canvas/*.test.*) — pass.
• Panel tests (src/features/panel/index.test.tsx) — pass.
• Rust system-prompt tests cover both the single-pane (no addendum) and multi-pane (titles + CLI commands + blocking contract) cases — pass.
• Manual: drag-split a conversation, resize/rearrange/close panes, ⌘W, reload to confirm persistence; send from a split and confirm the agent can list/read/message siblings.

Note: pre-existing, unrelated test failures on main in features/editor/index.test.tsx (Monaco focus/timing) and features/terminal/terminal-presets.test.ts, and one env-sensitive rust test (workspace::scripts ... helmor_port_vars), are not affected by this branch — verified they fail identically on a clean base.

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