feat(coding-agent): add host request contracts

[sethkarten]

Summary

• define host request context contracts
• add request contract coverage

Testing

• Not run (per task instruction)

Note

Add host request handler contracts with context validation and forgery prevention

• Introduces HostRequestContext interface in packages/coding-agent/src/core/kernel/index.ts providing per-call requestId, generation, AbortSignal, and isCurrent() for authority checks.
• Adds createHostRequestHandler factory that enforces two-argument implementations and validates context via assertGenuineHostRequestContext before delegating to user code.
• Adds assertHostRequestHandler guard that rejects forged handlers using a branded symbol plus WeakSet provenance tracking, preventing copied-symbol or raw-function spoofing.
• Tests in packages/coding-agent/test/host-request-contract.test.ts cover factory rejection of unary implementations, invalid context detection, and forgery prevention.

^{Macroscope summarized 15b56d0.}

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Note

Low Risk
Additive kernel types and test utilities only; KernelManager still dispatches unary hostHandlers and is not modified here.

Overview
Introduces staged, context-aware host-request handler contracts in the kernel module while the existing unary HostRequestHandler registration path stays unchanged for now.

New HostRequestContext carries per-call dispatcher authority (requestId, generation, AbortSignal, isCurrent()). createHostRequestHandler wraps two-argument implementations, validates context before user code runs, and brands handlers via a symbol plus a WeakSet so only factory-minted functions count. assertHostRequestHandler rejects forged handlers that copy the brand without provenance.

Adds host-request-contract.test.ts for unary rejection, invalid context, and forgery cases, plus host-request-context.ts test helpers to invoke handlers with synthetic dispatcher context.

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