[doc] chore: Add Codecov badge to README

[JoyboyBrian]

Added Codecov badge to README for coverage tracking.

What

Add Codecov badge to README

Why

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactor (code change that neither fixes a bug nor adds a feature)
• Documentation update
• CI/build configuration change

How to Test

Checklist

• PR title follows [module] type: description format
• Appropriate labels added (e.g. enhancement, bug, breaking)
• ruff check . and ruff format --check . pass
• pyright osmosis_ai/ passes
• pytest passes (new tests added if applicable)
• Public API changes are documented
• No secrets or credentials included

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Summary by cubic

Add a Codecov badge to the README to display current test coverage and link to detailed reports.

^{Written for commit d401fa5. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file