[DISTRIBUTION] Plugin marketplace — self-hosted marketplace.json + Anthropic submission

[urbantech]

Summary

• Adds `.claude-plugin/marketplace.json` — self-hosted AINative plugin marketplace listing the `zerodb-memory` plugin (self-referential, `source: "."`)
• Adds `docs/marketplace-submission.md` — ready-to-submit entry for `anthropics/claude-plugins-official`, with step-by-step PR instructions, install commands for both marketplaces, and a pre-submission validation checklist

Install commands

AINative marketplace (available immediately after merge):
```
/plugin marketplace add github:AINative-Studio/zerodb-claude-plugin
/plugin install zerodb-memory
```

After Anthropic approval:
```
/plugin install zerodb-memory@claude-plugins-official
```

Test Plan

• Verify `.claude-plugin/marketplace.json` is valid JSON and schema matches expected structure
• Confirm `source: "."` resolves correctly when used via `/plugin marketplace add github:AINative-Studio/zerodb-claude-plugin`
• Review `docs/marketplace-submission.md` submission entry JSON against `anthropics/claude-plugins-official` format requirements
• Walk through checklist items in `docs/marketplace-submission.md` to confirm all pass before submitting to Anthropic

Risk/Rollback

Low risk — documentation and metadata files only, no code changes. Rollback: revert this PR.

Closes #13, Closes #14