docs: improve skill discovery and developer guidance

[sent-dm]

PR title: docs: improve skill discovery and developer guidance

## Summary

- Add task-oriented catalogs for all 13 Sent skills across the repository, portable package, and generated adapter READMEs.
- Document Skills CLI installation, MCP capabilities, authorization, mutation safeguards, and the plugin development workflow.
- Expand discovery metadata for messaging, contacts, templates, analytics, and account-readiness skills.
- Add search-oriented plugin keywords covering SMS, WhatsApp, RCS, 10DLC, WABA, RBM, and deliverability.
- Add validation gates that prevent missing README catalog entries or weak skill descriptions.
- Regenerate the Codex and Claude Code adapters.

## Why

The plugin contained thorough skill workflows, but its READMEs did not make those skills easy to discover by task or search term. Developers also lacked a single reference for installation options, skill selection, MCP operations, safety behavior, and contribution requirements.

This brings the collection closer to the discovery and documentation patterns used by established Resend and Supabase skill repositories.

## Developer impact

Developers can now:

- identify the correct skill from natural-language examples;
- install individual skills or the complete collection;
- understand which capabilities require an authorized MCP connection;
- distinguish overlapping analytics, diagnosis, template, and onboarding workflows;
- follow a documented process for adding future skills without breaking discovery.

## Validation

- All 13 skills passed `quick_validate.py`.
- `npx skills add . --list` discovered all 13 skills.
- Repository validation passed.
- All 13 validation rejection cases passed.
- Fixture tests passed.
- Generated adapters match canonical sources.
- Strict Claude marketplace and plugin validation passed.
- Referenced documentation links returned HTTP 200.