refactor(task): split task spec from prompt execution

[kitlangton]

Summary

• keep task as a real registry-owned tool while splitting its registry-facing spec from its prompt-bound executor
• remove the direct SessionPrompt runtime dependency from tool/task.ts and bind the recursive task executor inside SessionPrompt
• preserve task visibility in ToolRegistry, debug tooling, and experimental endpoints without using dynamic registration or dynamic imports

Notes

• ToolRegistry still owns task metadata/spec (id, schema, description, and named/builtin visibility)
• SessionPrompt now owns the recursive task execution wiring by binding a runtime executor with cancel, resolvePromptParts, and prompt
• task execution tests now exercise the bound executor path explicitly

Testing

• bun typecheck
• bun turbo typecheck
• bun run test -- test/tool/task.test.ts test/session/prompt-effect.test.ts test/session/prompt.test.ts test/tool/registry.test.ts