[Plugins] Native Copilot adapter target metadata (skill/hook/extension, no execution) #1073

Part of #1068.

Scope

Allow plugin manifests to declare native Copilot adapter targets - Copilot skill, hook, or extension references - as metadata only in MVP. Squad records and surfaces them; Squad MVP does not load or execute them.

Requirements

• Add an adapters section to plugin.manifest.json supporting at least:
  • copilot.skill - name + path/ref to a Copilot skill definition the plugin contributes.
  • copilot.hook - name + trigger metadata.
  • copilot.extension - name + identifier.
• Validator enforces that adapter entries are well-formed and self-contained metadata.
• squad plugin list --adapters (or equivalent) surfaces declared adapter targets per installed plugin.
• Lock file records resolved adapter metadata for audit.
• Explicitly out of scope for this issue: loading, registering, or executing the adapter targets. That is gated by a future RFC.

Acceptance

• Manifest accepts and validates adapter declarations.
• list exposes them.
• Documentation clearly states "metadata only in MVP - no execution".
• Security gate (separate child) verifies no execution path is reachable from adapter declarations.

Planning provenance: derived from the epic's "native adapter targets" requirement.