feat: return promises from codemode tool calls

[rumpl]

Summary

• Return a Promise from every codemode tool call and execute tool handlers concurrently, resolving Promises on the single-threaded JavaScript event loop.
• Support top-level await, Promise.all, Promise.allSettled, and catchable tool failures while retaining ordered tool-call history and cancellation support.
• Update generated TypeScript declarations, model instructions, and documentation for the asynchronous API.
• Add tests covering parallel execution, sequential dependencies, Promise callbacks, rejection handling, cancellation, and unavailable handlers.

Existing scripts must await tool results before inspecting or combining them. Started tool calls finish before returning the script response unless execution is cancelled.

Validation

• task build — passed
• task lint — passed
• go test ./pkg/tools/codemode ./pkg/agent ./pkg/runtime — passed
• go test -race -count=20 ./pkg/tools/codemode — passed
• WebAssembly codemode tests — passed
• task test — blocked by TestUsageBackgroundRuntimeRecordsWithoutClientIO in pkg/acp; reproduced on unchanged main in a separate worktree. An initial DMR connection-test timeout passed on the next full-suite run.