[chore]: add .js to the end of all imports

[pirate]

why

• removes the need to append .js to all imports during esm build process
• aligns us with the default standard for esm / esbuild js projects that also need to support cjs
• this allows us to seamlessly support ESM, CJS, and TSX with no remapping needed

what changed

test plan

---

Summary by cubic

Standardized all internal imports and re-exports to include .js extensions to enable native ESM resolution, while keeping CJS and TS working without path remapping. Updated examples to use explicit index.js entrypoints and environment-driven config for easier local runs.

• Refactors

  • Appended .js to all relative imports and barrel exports across core, v3 agent, DOM runtimes/locator scripts, caches, handlers, external clients, and index files.
  • Switched examples to ../lib/v3/index.js, fixed nested re-export paths, and added env-based config (BROWSERBASE_API_KEY, BROWSERBASE_PROJECT_ID, MODEL_API_KEY) passed into Stagehand/model.
  • Updated dotenv to ^17.3.1 in dev deps. No public API changes.

• Migration

  • Most users importing from package entry points: no changes.
  • If importing internal files locally (e.g., examples or custom integrations), include .js in paths and prefer index.js for directories.

^{Written for commit d1d0547. Summary will update on new commits. Review in cubic}

[changeset-bot]

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Latest commit: d1d0547

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[greptile-apps]

Too many files changed for review. (399 files found, 100 file limit)

[cubic-dev-ai]

No issues found across 399 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

Note: This PR contains a large number of files. cubic only reviews up to 75 files per PR, so some files may not have been reviewed.