[Plugins] Knowledge and memory plugin pilot #1072

Part of #1068.

Scope

Pilot the plugin system with a real, useful capability: a knowledge / memory plugin that an agent consults at runtime.

Requirements

• Define the contract for knowledge and memory components in a manifest (purely declarative - files, indexes, lookup configs).
• Implement at least two pilot plugins backed by the same role so switching is meaningful (e.g., memory-local-files and memory-local-sqlite, or two knowledge packs of different content).
• Wire Squad so that an enabled memory/knowledge plugin is visible to agents during their runs (read path only - no plugin-supplied code execution).
• Document the contract so third-party authors can ship their own.

Acceptance

• Two pilot plugins install cleanly via squad plugin install.
• Switching between them produces a visible difference in agent output in a deterministic test.
• Contract documented in repo docs and linked from the epic.

Planning provenance: tamirdresher_microsoft/tamresearch1#2920.