docs: add minor merge and release skill

[Saadnajmi]

Summary

• Adds the repository merge-minor command and registration stub.
• Captures the topology, conflict-provenance, Jujutsu/Git, validation, evidence, and publication lessons from preparing the 0.84–0.87 stack.
• Treats prepared merge/release branches as recreatable baselines: this skill PR will evolve as the stack is reviewed, edited, rebased, and re-judged.

Scope

• .ai/commands/merge-minor.md — canonical operational instructions
• .claude/commands/merge-minor.md — command registration stub
• .ai/index.md — command index

Review workflow

This draft is based on 0.87-merge so changes learned while reviewing earlier stack branches can be layered here. Keep it draft until the stack review/rebase cycle is complete.

Validation

• Independent planner/executor/judge PASS
• Prettier and git diff --check pass
• Referenced repository scripts and documentation paths verified