feat: bridge Shift+Tab agent cycle from VS Code to webview (#878)

[jeonghun-jj-lee]

VS Code intercepts Tab keys before they reach the webview, so the in-app keybind alone is insufficient. This adds the VS Code→extension→webview→iframe bridge for the agent-cycle shortcut.

Extension side:

• package.json: amicode.cycleAgent command + shift+tab keybinding (when amicode.chat panel is active)
• extension.ts: command handler posts {kind: "agent-cycle"} to all live chat panels
• chat_panel.ts: agent-cycle added to both lane 2 relay allowlists

Fork side (harmoniqs/opencode#310):

• use-composer-commands.tsx: agent.cycle keybind → shift+tab, reverse → none
• app.tsx: DesktopCommands listens for agent-cycle bridge messages and triggers command.trigger("agent.cycle")

Test fix:

• mode_block.test.ts: session-creation timeout 60s → 120s (cold-start package install in temp HOME)

Closes #878
Depends on harmoniqs/opencode#310

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