wrangler build: add --write-workerd-config to emit workerd capnp config

[devin-ai-integration]

This PR adds a new --write-workerd-config [path] flag to the wrangler build command that generates a workerd capnp configuration file suitable for running the built Worker directly with the workerd binary.

Implementation Overview

• Architecture: The build command delegates to deploy --dry-run, so the implementation lives in the deploy command and only activates when both flags are present
• Config generation: Uses Miniflare's existing serializeConfig function to produce the capnp file
• Scope: Generates a minimal single-worker config with modules and compatibility settings (bindings like KV/R2/DO are not included in this initial version)
• Path handling: Supports both file and directory paths; defaults to workerd.capnp if a directory is provided

Key Implementation Details

The logic finds the built entry point by:

1. Checking common names (index.mjs, index.js, worker.mjs, etc.)
2. Falling back to scanning for any .js/.mjs file in the output directory
3. Throwing an error if no entry point is found

Human Review Checklist

High Priority:

• Test coverage: Verify the test adequately covers the feature (current test only checks file existence, not capnp validity)
• Entry point detection: Review the fallback logic for finding built files - is it robust enough?
• Path handling: Check edge cases around file vs directory paths, permissions, mkdir behavior

Medium Priority:

• Error scenarios: What happens if bundling fails or produces unexpected output?
• Config completeness: Is the minimal config (modules + compatibility only) sufficient for the initial version?
• Integration stability: Is the build → deploy --dry-run delegation approach sound?

Link to Devin run: https://app.devin.ai/sessions/f95b2210cf2b45788334302cb5ab93f5
Requested by: @petebacondarwin

---

• Tests
  • Tests included
• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because: flag is discoverable via --help; docs can follow in separate PR
• Wrangler V3 Backport
  • Wrangler PR:
  • Not necessary because: new feature targeting current Wrangler version

[devin-ai-integration]

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Latest commit: 8d0daba

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