Infinite Canvas — Phase 3: Universal connections + terminal routing

[Legacynnn]

Part of the Infinite Canvas epic: #61

Phase 3 — Universal connections + conversation→terminal routing

Let any panel connect to any other, form chains, and give connections real behavior.

Scope

• Connection model — canvas_connections table (id, workspace_id, from_panel_id, to_panel_id, kind, meta JSON) + CRUD commands + UiMutationEvent sync. (If created in Phase 1, wire it up here.)
• Connect UX — drag from a panel connector to another panel to create an edge; edges render on the canvas; chains allowed (A→B→C). Each panel exposes which panels it is connected to.
• Endpoint semantics:
  • Conversation → Terminal: agent shell commands run in / stream to that terminal.
  • Conversation → Conversation: orchestration link (dispatch/manage via Helmor CLI); visible on canvas.
  • Other pairs: scoped context binding (documented; deeper behavior can be incremental).
• "Run with a specific terminal" control — when a conversation is connected to ≥1 terminal, the user picks which terminal receives execution.

Out of scope

Browser/app endpoints (Phase 7), CLI-driven connect (Phase 6 — though the backend command should be reusable by the CLI).

Acceptance

• User can connect/disconnect panels and build chains; connections persist across restart.
• A conversation routes command execution to its connected terminal; "run with a specific terminal" selects the target.
• Each panel reports its connections in the UI.
• Connection persistence covered by tests; lint/typecheck/test pass.