feat(metadata): complete skill and marketplace discovery

[sent-dm]

What changed

• Added valid agents/openai.yaml metadata to the eight skills that lacked it.
• Enforced unique display names/descriptions and exact $skill-name invocation in every default prompt.
• Enforced non-mutating discovery prompts unless the owning skill has explicit mutation confirmation controls.
• Added one canonical marketplace content source under adapter-sources/shared/ and generated Codex/Claude metadata from it.
• Pointed the plugin homepage to the GitHub README.
• Balanced the marketplace description between MCP operations and specialist workflows.
• Exposed performance-analysis and 10DLC specialist starters while retaining readiness, analytics, and safe-send discovery in skill metadata.
• Added negative gates for missing metadata, prompt routing drift, duplicate names, and unsafe marketplace prompts.

Validation

• python3 scripts/generate_adapters.py --check
• python3 scripts/validate.py
• python3 scripts/test_fixtures.py
• python3 scripts/test_contracts.py
• python3 scripts/test_validation_gates.py
• Agent Skills validation for all canonical skills
• Strict Claude marketplace and plugin validation
• git diff --check

Stacked on the metadata PR.