[Plugins] Demo plugins and tests proving switching changes behavior #1074

Part of #1068.

Scope

Ship a small set of demo plugins and an automated test suite that proves switching plugins changes observable Squad behavior.

Requirements

• At least three demo plugins covering distinct component types (e.g., a docs/context-pack plugin, a governance-lite policy plugin, and one of the memory/knowledge pilots).
• Test fixtures install each demo plugin in a clean .squad/, run a deterministic Squad task, and capture output.
• Switching the active plugin (via squad plugin switch) must produce a different, asserted output for the same task.
• Tests run in CI and gate merges to prevent regressions in the switching contract.
• Demo plugins live under a clearly-marked examples/ or plugins/demo/ path so consumers can use them as templates.

Acceptance

• Demo plugins exist and install cleanly.
• CI test asserts behavior changes after switch.
• README in the demo directory explains how to reproduce locally.

Planning provenance: tamirdresher_microsoft/tamresearch1#2921, #2922, #2923, #2924, #2925.