Bootstrap AGENTS.md and agent documentation

[paulsmirnov]

Description

Replaces the monolithic .github/copilot-instructions.md with a compact, hub-and-spoke agent guidance architecture centered on AGENTS.md and specialized documentation in docs/agents/, CONTEXT.md, and DESIGN.md.

Key improvements:

• Reduces token footprint on every agent prompt / context cycle (~80% reduction in root instruction size).
• Establishes a modular documentation layout for architecture (docs/agents/architecture.md), code style (docs/agents/code-style.md), domain terminology (CONTEXT.md), design system (DESIGN.md), and issue triage workflow (docs/agents/issue-tracker.md, docs/agents/triage-labels.md).
• Retains an inline code completion cache in .github/copilot-instructions.md for non-agentic inline completion engines.
• Introduces living documentation files (docs/agents/lessons-learned.md) to capture domain patterns and common pitfalls.

Type of changes

• New feature (non-breaking change that adds functionality)

Checklist

• I have read CONTRIBUTING and CODE_OF_CONDUCT guides.
• I have followed the code style of this project.
• I have run yarn run ci: lint and tests pass locally with my changes.
• I have added tests that prove my fix/feature works OR The changes do not require updated tests.
• I have added the necessary documentation OR The changes do not require updated docs.

[NadezhdaKatcman]

No comments. Thank you!