Publish a plugin SDK, scaffold, and conformance test kit

## Parent and milestone

- Parent issue: #24
- Milestone: not scheduled; implementation depends on #14 and starts after #15.

## Problem

A protocol document alone will not make third-party integrations reliable or approachable. Developers need generated types, a minimal scaffold, examples, diagnostics, and tests that do not import Durex internals.

## Outcome

Publish a small developer kit that can create and validate a plugin, run protocol fixtures locally, and certify compatibility with a declared Durex plugin API version.

## Scope

- Provide JSON Schema for manifests and wire messages.
- Provide a reference SDK and language-neutral protocol examples.
- Add a `plugin init` scaffold design and a `plugin doctor`/validation path.
- Add conformance fixtures for lifecycle, cancellation, timeout, bounds, redaction, failure policy, and deterministic output.
- Document release, deprecation, and compatibility expectations.

## Non-goals

- Hosting a public marketplace.
- Trusting a plugin because it passes functional tests.
- Requiring third-party developers to use Python.

## Acceptance criteria

- [ ] A standalone example plugin passes without importing production modules.
- [ ] Diagnostics identify incompatible protocol versions and missing permissions.
- [ ] Documentation includes context provider, output filter, observer, and checkpoint examples.
- [ ] Targeted, failure-path, and conformance tests pass.
- [ ] Architecture, plugin-developer, configuration, and operator documentation is updated.

## Validation

- Run the conformance kit against the reference plugin and intentionally broken fixtures in CI.

## Dependencies

#33 and #34.

Start by reading parent issue #24 and the dependency notes for #14, #15, #33, and #34 to establish the protocol and implementation order. Define the standalone example plugin, scaffold, diagnostics, protocol fixtures, and documentation required by the acceptance criteria, then run the conformance kit against the reference and intentionally broken fixtures in CI.

Written by the indexing model from the issue text.