[Reborn] Add loop support services

[serrrfirat]

Parent

#2987

What to build

Add the Reborn loop support service layer behind AgentLoopHost: PromptContextService, MemoryPromptContextService, SkillContextService, InstructionBundleBuilder, and HostManagedModelGateway.

These services should assemble model input and model streaming through host policy while preserving Reborn authority boundaries. Loops should not own raw provider clients, raw memory repositories, raw secrets, or direct tool/runtime handles.

Run profile / model-context dependency

Run profile resolution is tracked in #3107. Loop support services should consume resolved model/context/checkpoint policy ids from the immutable per-run snapshot; the model gateway resolves effective provider/model per call within the model_profile_id policy envelope.

Acceptance criteria

• Instruction bundles can be built from thread snapshots, system/user/agent/project scope, memory context, skill context, visible capability surface, and safety policy.
• Memory prompt context uses Reborn memory services and respects tenant/user/agent/project isolation and current primary-scope rules.
• Skill context selection remains deterministic and does not bypass trust/capability visibility decisions.
• Model streaming routes through a host-managed gateway with provider/model policy, budget/resource accounting hooks, credential handling, redaction, and stable sanitized errors.
• Tests cover empty context, memory unavailable, skill unavailable, denied provider/model policy, credential missing, redaction sentinel, and deterministic bundle rebuild.
• The loop support layer does not call RuntimeDispatcher, raw provider clients, raw secret stores, or raw filesystem/network/process handles directly.

Blocked by

• Blocked by [Reborn] Reborn cutover blocker: add reference AgentLoopHost facade #3016 for the AgentLoopHost facade contract.
• Coordinate with feat(reborn): add memory document storage boundary #3078 and feat(reborn): add memory search and plugin seams #3079 for Reborn memory storage/search services.
• Coordinate with [Reborn] Add ToolSurfaceService and CapabilityCatalog #3090 for ToolSurfaceService / CapabilityCatalog.

[serrrfirat]

Related design issue created: #3107.

Loop support services should consume the resolved run profile/run class, but still remain host-owned and loop-neutral.

Examples:

interactive_coding -> lightweight driver, tight capability surface, short checkpoints
long_running_mission -> CodeAct/planner driver, durable checkpoints, mission guidance steering

InstructionBundleBuilder, PromptContextService, SkillContextService, and HostManagedModelGateway can use the resolved profile to choose prompt overlays/model policy/context strategy, but loops still receive only policy-filtered bundles and never raw memory repos/provider clients/secrets/tools.

This keeps variation in prompt/model/context policy while avoiding loop-kind branching in lower authority/runtime services.