feat(agent): add governed Codex runtime and tool contracts

[buger]

Consolidates the governed Codex/Luna execution chain on the canonical Probe product branch.

Scope:

• governed Luna profile and Codex runtime lifecycle
• typed answer contract
• dispatch binding and requested/observed result identity
• canonical governed MCP tool schemas shared by discovery and execution
• privacy-bounded typed tool start/terminal progress events
• public @probelabs/probe/agent/mcp BuiltInMCPServer export with adjacent NodeNext declarations
• public readonly ProbeAgent.abortSignal used by governed MCP tools with no private-controller fallback
• privacy-safe SHA-256 digests of schema-validated governed tool arguments across lifecycle events, without raw argument or result exposure

Evidence:

• constituent zero-model focused tests passed for the governed profile, runtime/answer, MCP, and P0 tool-contract/progress slices
• P0 recorded 7/7 focused progress, 7/7 profile, 48/48 governed answer, and 22/22 governed MCP tests
• the public MCP export slice passed its focused native 8/8 test, including package self-reference and NodeNext consumer compilation
• the public-only facade cancellation contract passed focused native 8/8, including root and agent NodeNext consumers, exact signal injection, failed progress, and awaited close
• the validated-argument digest slice passed focused native 11/11, including canonical framing/default behavior, lifecycle privacy, failure containment, cancellation, public package use, and NodeNext consumer compilation
• this evidence remains provisional at the consolidated head; full consolidated tests are still pending

This PR remains a draft and does not claim production admission.

North-star program: https://github.com/probelabs/reqproof/pull/1035