docs: correct architecture docs after the platform->infrastructure rename

[YauhenBichel]

What

Fixes accuracy gaps in the architecture docs that remained after the
platform -> infrastructure rename (#5295) and the gateway turn-handler
unification. Documentation only — no code changes.

Why

An audit of the architecture docs against the current code found the rename was
almost fully propagated, but a few concrete defects and gaps survived: a wrong
tier count, a stale mermaid identifier, an undocumented core subpackage, and a
peer rule that no longer matches the enforced import graph.

[github-actions]

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[YauhenBichel]

@greptile review

[greptile-apps]

Greptile Summary

The PR updates architecture documentation after the platform to infrastructure rename and shared turn-handler unification.

• Corrects the documented layer count from four to five.
• Replaces the stale Mermaid PLATFORM identifier with INFRA.
• Documents gateway transports, middleware, web responsibilities, and infrastructure/turn_host.
• Records the three tracked tier-1 import exceptions.

Confidence Score: 5/5

The documentation-only PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
AGENTS.md	Corrects the architecture-document description to identify a five-tier layer table.
docs/ARCHITECTURE.md	Updates renamed infrastructure identifiers and documents current gateway, middleware, turn-host, and import-boundary architecture.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    HOSTS["Tier 1: surfaces + gateway"] --> BOOT["Tier 2: bootstrap"]
    BOOT --> CAP["Tier 3: tools + integrations"]
    CAP --> RUNTIME["Tier 4: core + infrastructure"]
    RUNTIME --> CONFIG["Tier 5: config"]
    CORE[core] <--> INFRA[infrastructure]

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[github-actions]

🛸 Aliens watching our repo just upgraded @YauhenBichel's threat level to: do not engage — too competent. 👽

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