feat(sdk): add provider-neutral authorization seam

[EnzoTironi]

Summary

Adds an optional provider-neutral AuthorizationProvider seam to Executor tool execution so hosts can enforce a horizontal PDP without coupling Executor core to Cerbos, OPA, or another policy engine.

The provider receives Executor-bound tenant/subject identity plus resolved tool metadata and arguments. It is consulted after Executor's existing hard block checks and before credential resolution or plugin invocation.

Behavior:

• no provider configured: existing behavior is preserved;
• allow: execution continues normally;
• deny: fails closed before credentials or plugin invocation;
• provider failure: fails closed;
• require_approval: reuses Executor's existing elicitation flow;
• nested ctx.execute(...): re-enters the same authorization seam;
• caller arguments cannot mint tenant/subject identity;
• the real scoped host threads the optional provider through HostConfig to createExecutor.

No concrete PDP dependency is added. Product/host code owns the provider implementation.

Validation

Validated against current upstream main with the branch 0 commits behind before publication:

• packages/core/sdk/src/authorization.test.ts: 10/10 passing
• packages/core/api/src/server/scoped-executor.authorization.test.ts: 1/1 passing
• @executor-js/sdk typecheck: passing
• @executor-js/api typecheck: passing
• repository bun run lint: 0 warnings, 0 errors
• changeset included for the public release surface

The seam is also exercised by an external integration acceptance using a provider implementation outside Executor core, confirming the intended host-owned composition model.

[EnzoTironi]

Closing this PR because this authorization seam is intended to live in our maintained fork rather than be proposed to upstream. The fork main is synced exactly to upstream main before applying the patch.