docs(plugins): document plugin contract v2, the inventory and ADR 0034

[thymikee]

Summary

Part 3 of 4 of #3755 (umbrella #3747), stacked on #3768. Blocked by #3748: do not merge before it.

Docs only:

• website/docs/docs/plugins.md: v2 authoring (./agent-device export, factory with options, setup, credentials with byBackend, connection), entry discovery and the no-fallback rule, the registry order and duplicate rule, daemon status and the inventory entry shape, and a short section on v1 plugins.
• ADR 0034, "Provider plugin registry" (brief): three sources, one shape, set fixed per daemon, failure isolation, and a short daemon-ownership note pointing to feat(client): explicit daemon lifecycle with ensureDaemon() and attach clients #3754 and feat(host): startAgentDevice() for embedder-owned daemons #3757. Indexed in docs/adr/README.md.
• CHANGELOG.md entry for the contract, registry, isolation, inventory and daemon status.
• limrun.md and build-an-integration.md follow the renamed diagnostic and contract.

Runtime tests do not apply to prose; the docs checks in check:affected ran.

Validation

pnpm check:affected --run --head 495046c784 (the stack top, which contains this layer): passed (exactCommit).

🤖 Generated with Claude Code

[github-actions]

Size Report

Metric	Base	Current	Diff
Installed (including dependencies)	5.35 MB	5.35 MB	0 B
Package (unpacked)	5.35 MB	5.35 MB	0 B
Package (download)	1.61 MB	1.61 MB	-9 B

Startup median (7 runs, lower is better):

Scenario	Base	Current	Diff
CLI --version	17.8 ms	18.2 ms	+0.4 ms
CLI --help	49.3 ms	50.3 ms	+1.0 ms

[github-actions]

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[cubic-dev-ai]

All reported issues were addressed across 6 files

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[thymikee]

This PR is ready at 058826d. CI is green, with 14 checks and none failing, and the change is docs-only. No conflicts. Not blocking, take or leave: the Connect section at website/docs/docs/plugins.md#L95 lists resolve({flags,env,cwd,stateDir}) and verify({flags,env}), but the contract in packages/plugins/src/contract.ts also passes host to both callbacks, so the example could add it; and the CHANGELOG.md entry is one long bullet that mixes contract, registry, status, fingerprint and Limrun changes, so a few shorter bullets would read better. I compared the docs against the base branch head (5645905), not a checkout of this PR head. I did not run the docs build, trace the registry ordering or the plugins list inventory behavior into code, or check the ADR header format against ADRs 0001-0033. Nothing else stands in the way; the stack merges in order (#3767, #3768, then this PR).

[thymikee]

Thanks. Taken in 5c46bf5: the Connect example now passes host to resolve and verify, with a sentence saying it is the same ProviderPluginHost that setup gets, and the CHANGELOG entry is split into five bullets (contract, registry, failure isolation and status, client fingerprint and Limrun fingerprint, partial-Limrun diagnostic).