Add .github/copilot-instructions.md with coding conventions

[dgenio]

What changed

• Created .github/copilot-instructions.md (98 lines) with repository-level coding guidance for GitHub Copilot
• Added [tool.ruff.lint.isort] / known-first-party = ["chainweaver"] to pyproject.toml to back the import-order claim in the instructions file (behavior-preserving — codifies Ruff's auto-detection)

Sections included

Section	Purpose
Language & runtime	Python 3.10+, from __future__ import annotations, type annotations
Code style	Ruff config summary (matches pyproject.toml exactly)
Architecture rules	Pydantic models, exception hierarchy, no LLM in executor
Project layout	Key files/dirs so Copilot can orient without searching
Testing	pytest conventions, fixtures, assertion style
Validation commands	ruff check, ruff format --check, mypy, pytest (matches CI)
PR conventions	Imperative mood titles, one logical change per PR
Anti-patterns	Explicit "never generate these" rules
Trust these instructions	Per GitHub best practices — tells Copilot to defer to this file

Why

Closes #53

Without this file, Copilot has no project-specific guidance on coding style, testing patterns, import organization, or validation commands. This complements AGENTS.md (architecture context) with coding conventions scoped to Copilot specifically.

How verified

• ruff check chainweaver/ tests/ examples/ — all checks passed
• ruff format --check chainweaver/ tests/ examples/ — 13 files already formatted
• python -m pytest tests/ -v — 46 passed, 0 failed
• Line count: 98 lines (within ≤120 limit)

Acceptance criteria checklist

• .github/copilot-instructions.md exists
• File is ≤120 lines (98)
• File links to AGENTS.md for architecture context (header + footer)
• Ruff config cited matches pyproject.toml [tool.ruff] section
• Validation commands match CI pipeline (.github/workflows/ci.yml)
• No architecture map or decision context (those live in AGENTS.md)
• Anti-patterns section included

Tradeoffs / risks

• Drift risk: If Ruff rules change in pyproject.toml, this file must be updated in the same PR. This convention is documented in the file itself (PR conventions section).
• Project layout addition (Mode B delta): Added a project layout section not in the original spec. This follows GitHub's official recommendation to include layout info so Copilot doesn't have to search the codebase each time. Stays well within the 120-line budget.

Scope notes

• .github/copilot-instructions.md — new documentation file (98 lines)
• pyproject.toml — added [tool.ruff.lint.isort] with known-first-party = ["chainweaver"] to explicitly back the import-order claim in the instructions file. This is behavior-preserving (codifies Ruff's existing auto-detection).

[Copilot]

Pull request overview

Adds repository-scoped GitHub Copilot guidance for ChainWeaver to help generated code match the project’s Python/runtime constraints, style/tooling, layout, testing conventions, and PR hygiene.

Changes:

• Introduces .github/copilot-instructions.md with language/runtime rules, Ruff/mypy/pytest commands, architecture constraints, and anti-patterns.
• Documents project layout to reduce agent search and improve navigation context.
• Establishes Copilot-specific PR conventions and “trust these instructions” guidance.

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