Add Cohesivity Sites third-party plugin

[AragAgg]

Summary

• Adds a self-contained catalog/cohesivity-sites/ third-party connector plugin that provisions temporary backends (hosting, Postgres, object storage, email, AI inference) for agent-built apps via Cohesivity's HTTP API.
• No Core files changed. Plugin includes an agentlas.plugin/v1 manifest, a SKILL.md with the full agent workflow, and a README.md.
• No account, OAuth, or CLI needed — agents bootstrap an ephemeral tenant with npx @cohesivity/init@0.8.2 --tenant-only and call the API directly. Tenants expire after 72 hours unless claimed.

Change type

• Third-party service or connector

Third-party integration boundary

• I read PLUGIN_CONTRIBUTIONS.md.
• The integration is independently installable, disableable, and removable.
• This pull request adds no provider-specific Core registry, CLI, loadout,
  credential, diagnostic, ranking, routing, allowlist, or runtime-mirror
  branch.
• Activation is explicit; the integration is not added to default, full,
  automatic, fallback, or background execution.
• Capabilities, auth type, hosts, operations, scope, and privacy effects are
  declared without credential values.
• Missing credentials, denied permissions, offline state, and provider
  failures are bounded and actionable.

Public safety and verification

• I staged only an explicit public allowlist.
• No secrets, environment files, private paths, logs, tests, fixtures,
  benchmark assets or results, screenshots, or internal plans are included.
• I ran the relevant verification commands from README.md.
• Verification: scripts/verify-package.sh is not applicable — this PR adds only catalog metadata and a skill file, no Core package changes.

Verification notes

Three files added under catalog/cohesivity-sites/:

• plugin.json: manifest with third-party-connector architecture, declared capabilities, auth fields, allowed hosts, privacy disclosure, and lifecycle behavior.
• skills/sites/SKILL.md: step-by-step agent workflow — bootstrap, read docs, provision, build, deploy, verify, update. Adapted from a tested Agentty integration.
• README.md: install and requirements summary.

The same skill flow has been validated end-to-end in Agentty (separate integration), including tenant creation, hosting/database/storage provisioning, live deploy, and independent HTTP verification.

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