0.22: remove superseded plugin, client and environment APIs #3759

Part of #3747. The breaking removals for 0.22. The non-breaking preparation landed in 0.21.x (see "Done in 0.21.x" below).

Each section below is one PR, and each stays under 2,500 changed lines.

1. Plugin v1 (est. 900–1,500 lines)

• Remove the v1 loader adapter (packages/plugins/src/legacy.ts), the v1 discovery and store branches, the manifest credential and capability fallbacks, and the install-time v1 collision checks.
• Keep package-export resolution, entry containment checks and registry validation.
• @agent-device/testmu: remove the v1 entry (src/plugin.ts), its build entry and the agentDevicePlugin manifest declaration.
• Migration note for plugin authors: v1 manifest → v2 AgentDevicePluginFactory.

2. Managed clients (est. 1,500–2,300 lines)

• createAgentDeviceClient defaults to daemonLifecycle: 'attach'.
• Remove managed and the client transport's implicit start, replace and timeout-kill paths.
• Keep explicit ensureDaemon(), quiesce, CLI/MCP ensuring, remote attachment, and the shared startup helpers they still use.
• Migration note for library clients: call ensureDaemon() for a detached daemon, or startAgentDevice (imported from agent-device) to own one in-process.

3. Ambient SDK defaults (est. 900–1,600 lines)

• Remove the process captures in src/sdk/host.ts. Released SDK functions then need an explicit host: Android adb, artifacts, Metro, remote-config and the public Limrun construction.
• resolveRemoteConfigProfile requires env, and createLocalArtifactAdapter requires cwd.
• Review refactor: read host settings from HostEnvironment in root src and remaining packages #3751's waived factory boundaries one by one.
• Remove the SDK entries from R86 PROCESS_ENTRIES (scripts/layering/process-globals-policy.ts).
• Keep the executable captures and startAgentDevice's documented optional-host capture.

4. Built-in provider special cases

• After refactor(provider-limrun): ship Limrun as the @agent-device/limrun plugin #3311 extracts Limrun, remove the built-in Limrun first-claim special case (src/builtin-provider-plugins.ts). Keep the registry's first-claim semantics. This is tracked in refactor(provider-limrun): ship Limrun as the @agent-device/limrun plugin #3311.

Done in 0.21.x (non-breaking)

• startAgentDevice from the main agent-device entry (feat(sdk): start an embedded runtime from the main entry #3843).
• The embedded runtime's owner closes it: daemon stop refuses embedded runtimes (refactor(daemon): close embedded runtimes only from their host #3850).
• TestMu's v2 plugin no longer reads capabilities from its v1 manifest (refactor(daemon): close embedded runtimes only from their host #3850).

Done when

• The 0.22 docs and release notes carry migration notes for plugin authors (v1 → v2), library clients (ensureDaemon() or startAgentDevice from agent-device), and CLI/MCP users.
• There are changesets for every affected package.