[codex] Add MIT license file and planning workflow docs

[roackb2]

Summary

• Add a root LICENSE file containing the MIT License text.
• Update the README license section to point readers to the license file.
• Document the feature PRD to technical plan workflow in docs/agent-context.md so future agents know when to consult it.

Impact

This keeps Heddle's published package metadata and user-facing repository documentation aligned around MIT licensing, while also preserving the current agent workflow guidance that was already staged locally.

Validation

• Searched the README, docs, package metadata, and LICENSE file for stale Apache references.
• Reviewed the staged docs/agent-context.md diff before including it in the PR.
• Confirmed the PR branch now contains both the MIT license update and the agent context documentation update.