chore: add openspec workflow

[dkarter]

Motivation

Add OpenSpec as a living spec layer for bullets.nvim and keep specs tied to the existing Plenary/Busted test suite.

Summary of Changes

• Add OpenSpec capability specs for current plugin behavior.
• Tag active tests with scenario IDs and add focused mise run test:spec <id> execution.
• Add spec/test link validation, coverage/report tasks, and CI enforcement.
• Document the OpenSpec workflow in AGENTS.md.

Testing

• mise run openspec:check
• mise run spec:coverage --min-coverage-percent 100
• mise run fmt:check
• mise run lua:lint
• mise run test

Dependencies/Special Considerations

All commits on the branch contain SSH signatures in the raw commit objects. Local signature verification requires gpg.ssh.allowedSignersFile to be configured.