Add a SpecKit configuration and AGENTS.md

[freakboy3742]

An initial set of LLM compatibility configurations. This is primarily for discussion purposes.

Adds:

1. An initial set of SpecKit templates
2. A SpecKit project constitution
3. OpenCode integration with SpecKit
4. An AGENTS.md for general LLM interactions

The only two files that have human-generated content are .specify/memory/consistution.md and AGENTS.md. The rest are automatically generated by the SpecKit install process (specify init . --force --ai opencode). It's not 100% clear to me if all these files should be committed to the repository, or if specify init should just be a "local environment setup" thing, and the .opencode and most of the .specify folder should be excluded from version control. It certainly appears as if they could be excluded.

When a user engages in an SpecKit session, it generates a /specs folder with the research and plan. My understanding is that in "pure" spec driven development, these should be committed, as they become the living specification that will be fed into future specifications. I haven't included the specifcations generated for #2732 as I'm not entirely sure I was using the tools correctly (i.e., I did lots of manual modification, and I'm not certain the final product actually reflects the specification).

The initial drafts of the constitution and AGENTS.md were generated with Claude; modifications have also been generated by interaction with Claude.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct