feat: add ecosystem plugin system

[coopbri]

Description

Add ecosystem plugin system that lets omni <product> <args> delegate to product CLIs, API adapters, and app launchers via discoverable plugin manifests.

• Plugin manifest types with TOML parsing (bin, api, launch)
• Plugin discovery from ~/.config/omni/plugins/ with PATH fallback
• Plugin execution for bin and launch types
• CLI routing: omni plugins, omni install, omni uninstall, external subcommand delegation
• Integration tests and example beacon plugin manifest
• Changeset for minor version bump

Test Steps

• Run cargo test plugin -- all unit and integration tests pass
• Run cargo clippy -- -D warnings -- clean
• Create a test plugin:

  mkdir -p ~/.config/omni/plugins/hello
  cat > ~/.config/omni/plugins/hello/plugin.toml << 'TOML'
  name = "hello"
  version = "0.1.0"
  description = "Test greeting plugin"
  type = "bin"
  bin = "echo"
  TOML

• Verify omni plugins lists the hello plugin
• Verify omni hello world prints "world"
• Clean up: rm -rf ~/.config/omni/plugins/hello

[changeset-bot]

🦋 Changeset detected

Latest commit: 65db92a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@omnidotdev/cli	Minor

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