refactor(plugins): extract the provider plugins into a private package

[thymikee]

Summary

Extract src/plugins (manifest, store, host, load, connection) into the private @agent-device/plugins package. 30 files, +172/−66.

• ProviderPluginHost is owned by the package; agent-device/plugins re-exports it unchanged.
• RemoteConfigProfile is declared once in contracts/remote-config-fields and re-exported by src/remote/remote-config-schema.ts.
• The loader reads CloudWebDriverProviderOptions from provider-webdriver/plugin; dynamic imports and cleanup are unchanged.
• The plugins layering zone stays rank 4. No allowlist or baseline change.
• The reserved-provider assertion moved to the root provider-device-runtimes test, which owns that composition.

Remote overlap: #3711 (unmerged) edits src/remote. This PR touches only remote-config-schema.ts and plugins/connection.ts type imports; reconcile on whichever lands second.

Validation

Head 03f44c4f11 on b7c93e3a37. Pinned pnpm 11.17.0, under the shared extraction lock.

• AGENT_DEVICE_VITEST_MAX_WORKERS=1 pnpm check:affected --run: exit 0 (1852 files, 15581 tests passed, 1 skipped).
• Also green: pnpm typecheck, pnpm depgraph, pnpm check:layering, pnpm check:production-exports, pnpm check:fallow, focused plugin tests.
• Earlier runs with default workers failed platform-android/fill-verification.test.ts:144. That file is untouched by this PR, and the same failure reproduces on main at 6e0844ff. It did not recur at one worker; I did not fix it here.
• Not run: CI Integration and Coverage jobs.

[cubic-dev-ai]

No issues found across 30 files

_{View guided diff | Turn on auto-fix | Re-trigger cubic}

[github-actions]

Size Report

Metric	Base	Current	Diff
Installed (including dependencies)	5.29 MB	5.29 MB	+256 B
Package (unpacked)	5.29 MB	5.29 MB	+256 B
Package (download)	1.59 MB	1.59 MB	+53 B

Startup median (7 runs, lower is better):

Scenario	Base	Current	Diff
CLI --version	28.2 ms	27.9 ms	-0.2 ms
CLI --help	78.7 ms	83.4 ms	+4.7 ms

[github-actions]

PR Preview Action v1.8.1
Preview removed because the pull request was closed.
2026-10-16 13:47 UTC

[thymikee]

This PR is ready at 03f44c4. The provider plugin extraction looks correct, and I found no code problems. Not blocking, and fine to take or leave: the lockfile diff adds 24 unrelated deprecated: yuku-codegen/yuku-parser lines beside the new packages/plugins importer (regenerating with the pinned pnpm would keep only the @agent-device/plugins entries), and remote-config-schema.ts now only passes RemoteConfigProfile through, so src/sdk/remote-config.ts could import it from @agent-device/contracts/remote and the pass-through could go, or that can wait for the #3711 reconciliation. I did not run the build or the installed-package integration test, and I did not rerun the local gates you reported. Both Smoke Tests jobs were still pending when I checked, and the diff touches that route only through bundling of the @agent-device/plugins/load import. No conflicts. Smoke Tests need to finish green before merge, and then the remote-config-schema.ts overlap with #3711 should be reconciled in whichever PR lands second.