docs: add OpenTag project link to README

[serenakeyitan]

Replaces #2395 after renaming the source branch to satisfy the repository branch-name convention.

Summary

• add a README link to the OpenTag project
• remove trailing whitespace from the edited line

Validation

• git diff --check
• docs-only change; full local test suite not required by AGENTS.md

Change Surface

• docs or contributor-facing repository metadata
• apps/cli public CLI or help output
• tree onboarding / binding / inspection behavior
• shipped or planned skill topology
• CI / packaging / release plumbing

Notes

• No package, install, runtime, or deployment behavior changes.

[yuezengwu]

Reviewed exact head d17747f072d2a252c54d0da766b0cc588ac44860 against main f8c424f1875c9f901a86e60147ffc466e559e438. The change replaces the top README callout with an OpenTag project link. The Markdown link resolves to the public OpenTag repository, and its README documents bringing your own Claude or Codex subscription.

No blocking findings. git diff --check and the isolated merge check pass; current CI checks are success or skipped. No product tests were run for this documentation-only review.