Raw request body access for plugin routes (webhook signature verification)

[marcusbellamyshaw-cell]

Proposal to close #1735: plugin routes have no way to access the raw, unparsed request body, which blocks HMAC-style webhook signature verification (Stripe, GitHub, Shopify, etc.) inside a plugin route -- those schemes sign the exact raw bytes, and re-serializing ctx.input does not reproduce them.

Proposed API

Additive opt-in on the route descriptor:

export interface PluginRoute<TInput = unknown> {
  input?: z.ZodType<TInput>;
  raw?: boolean; // new
  public?: boolean;
  handler: (ctx: RouteContext<TInput>) => Promise<unknown>;
}

export interface RouteContext<TInput = unknown> extends PluginContext {
  input: TInput;
  rawBody?: string; // new -- populated when the route sets raw: true
  request: Request;
  requestMeta: RequestMeta;
}

Setting raw: true skips the unconditional request.json() call in both handlePluginApiRoute (trusted/configured plugins) and handleSandboxedRoute (sandboxed/marketplace plugins), reads the body once via request.text(), and populates ctx.rawBody instead of ctx.input. Routes that don't set raw see zero behavior change -- same parse, same ctx.input, same guardConsumedRequestBody proxy (added for #1293).

Out of scope

• Built-in HMAC verification helpers for specific providers -- this is only about exposing the raw bytes; verification is the plugin's job.
• Streaming/chunked raw body access for very large payloads -- webhook payloads are KB-sized, request.text() is sufficient.

Acceptance

• PluginRoute.raw / RouteContext.rawBody typed and documented.
• raw: true skips request.json(), populates ctx.rawBody, for both trusted and sandboxed paths.
• Regression test proving non-raw routes are unaffected.
• Integration test: a raw: true route computes an HMAC over ctx.rawBody matching a signature computed independently over the same request bytes.
• Changelog entry under Plugin API.

Full writeup with exact call-site references (emdash-runtime.ts L3132/L3381, routes.ts L33-49, types.ts L1061-1083) is on the issue. Happy to open the PR once the shape looks right.