AD-501: Define runtime adapter interface

[alph-cmky]

Summary

• Adds a shared RuntimeAdapter contract for Codex, Claude, and shell-style execution.
• Defines adapter capabilities for executeTask, cancelTask, and getCapabilities.
• Defines streamed task events for stdout, stderr, tool events, diff, completion, and failure.
• Exports execution contracts from @agentdeck/runtime-adapters.
• Marks AD-501 complete in the development backlog.

Stack

• Stacked on AD-402: Add chat workspace API #38 (feat/ad-402-chat-workspace-api).

Verification

• TDD red check: pnpm test packages/runtime-adapters/src/execution/runtime-adapter.test.ts packages/runtime-adapters/src/execution/task-events.test.ts initially failed because runtime-adapter.js and task-events.js did not exist yet.
• pnpm test packages/runtime-adapters/src/execution/runtime-adapter.test.ts packages/runtime-adapters/src/execution/task-events.test.ts
• pnpm lint && pnpm format && pnpm typecheck && pnpm test && pnpm build

Closes #14