feat: structured RPC route

[0xpolarzero]

Warning

This PR is part of a stacked PR chain. Review/merge in order:
#144
> #145
#143
#147

Overview

Adds a structured RPC endpoint for command execution. This is the transport base for the TypeScript client (implemented in #143).

API

POST /_incur/rpc
content-type: application/json

{
  "command": "project deploy",
  "args": { "id": "p1" },
  "options": { "dryRun": true }
}

Changes

• Add POST /_incur/rpc for structured command calls.
• Add structured command execution so RPC receives args and options separately.
• Return validation fieldErrors from HTTP command responses.
• Document the RPC endpoint in the README.
• Add a changeset.

Tests

• Adds RPC route tests for execution, shape validation, and schema validation errors.

[0xpolarzero]

Will reopen better PRs after playing around and figuring out the best API.