feat: sample plugin

[hieu-w]

Summary

• Adds plugins/sample, a reference plugin moved from the standalone agent-wallet-plugin-template repo into this examples monorepo.
• Demonstrates the full plugin API: input schemas (ping), wallet-read / wallet-submit, and reserved capability declarations (mnemonic-read, config-write, network-manage).
• Follows the same workspace layout as ens (portal devDependency to @metamask/agent-wallet, yarn build + oclif manifest).

Commands

Command	Capability	Purpose
mm ping [name]	none	No-auth greet; input schema + positional args
mm demo balance	wallet-read	Active address, native balance, tx count
mm demo submit	wallet-submit	Obtain ctx.walletExecutor
mm vault mnemonic	mnemonic-read (reserved)	Declares SRP disclosure; SRP stays host-only
mm admin config	config-write (reserved)	Declares config-write capability
mm admin network	network-manage (reserved)	Declares network-manage capability

Test plan

• yarn install && yarn build — both ens and sample compile and generate oclif manifests
• yarn workspace sample run type-check
• yarn setup then mm ping Alice
• mm demo balance (requires login + init)
• mm plugins uninstall sample

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