Add practical Weave Git merge skill

[leynos]

Summary

This branch adds a practical Weave Git merge skill so agents can configure,
inspect and troubleshoot the driver from its actual implementation rather
than relying on the project's limited user documentation.

The guidance explains Git's per-file driver contract, Weave's entity-level
resolution pipeline, fallback boundaries, exit statuses and safe inspection
during merges and rebases.

Review walkthrough

• Start with the core skill workflow for configuration, preview, conflict inspection and recovery guidance.
• Then review the code-backed behaviour reference for the resolution pipeline, clean and conflicted cases, fallbacks and supported setup patterns.
• Finish with the skill metadata for discovery and the default invocation prompt.

Validation

• Skill structural validator: passed.
• git diff --check origin/main...HEAD: passed.
• Context-isolated forward test against a stopped-rebase troubleshooting scenario: passed; the resulting stage-inspection omission was corrected.
• make all: formatting, lint, type checking and all 187 tests passed. The repository-wide spelling step remains red only for pre-existing findings in unrelated skills; the new weave-git-merge skill has no spelling findings.

Notes

The skill was derived from the Weave driver, setup command, merge engine,
fallback implementation and integration tests in the local Weave checkout.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch weave-skill

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