docs(architecture): add comprehensive architecture documentation

[zbigniewsobiecki]

Summary

• Add a hub-and-spoke architecture documentation suite in docs/ARCHITECTURE.md + docs/architecture/ covering all backend subsystems (excluding tests and dashboard web frontend)
• Hub document provides system overview with Mermaid diagrams (architecture + end-to-end sequence) and links to 10 deep-dive documents
• Deep-dives cover: services, webhook pipeline, trigger system, agent system, engine backends, integration layer, gadgets, config/credentials, database, and resilience
• All claims in documentation were fact-checked against source code and corrected where needed
• Add 36 unit tests validating document existence, expected sections, and cross-reference integrity
• Remove stale biome-ignore suppression in project-harness-form.tsx (function no longer triggers complexity rule)

Documents

#	Document	Content
Hub	ARCHITECTURE.md	System overview, Mermaid diagrams, patterns, directory map, TOC
01	01-services.md	Three services, startup sequences, worker container model
02	02-webhook-pipeline.md	Handler factory, platform adapters, 12-step pipeline
03	03-trigger-system.md	TriggerRegistry, handlers, built-in triggers, config resolution
04	04-agent-system.md	YAML definitions, profiles, capabilities, prompts, hooks
05	05-engine-backends.md	AgentEngine interface, archetypes, execution adapter
06	06-integration-layer.md	IntegrationModule, registry, categories, providers
07	07-gadgets.md	Capability-to-gadget mapping, built-in tools, cascade-tools CLI
08	08-config-credentials.md	Config provider, credential resolution, encryption
09	09-database.md	ER diagram, tables, repositories, migrations
10	10-resilience.md	Watchdog, concurrency, rate limiting, retry, loop prevention

Test plan

• 36 unit tests validate all docs exist with expected sections and cross-references
• npm run lint passes (only pre-existing complexity warning in webhooks.ts)
• npm run typecheck passes
• Pre-commit hooks pass (lint + typecheck + commitlint)
• Pre-push hooks pass (unit tests)

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