chore: Add AI instructions

[Copilot]

This PR replaces the minimal and incorrect copilot instructions file with comprehensive, validated instructions for working with the duckdb-r repository. The previous file incorrectly referenced "dm Package" instead of duckdb-r and provided no useful guidance.

Key Improvements

Validated Commands and Timing: All build and test commands have been executed and timed to provide accurate expectations:

• First-time build: 10-15 minutes (validated at 13 minutes with parallel compilation)
• Subsequent builds: ~3 seconds (with cache)
• Full test suite: ~45 seconds (5800+ tests with 1 expected failure)

Critical Timeout Warnings: Added explicit "NEVER CANCEL" warnings with specific timeout values (30+ minutes for builds, 5+ minutes for tests) to prevent premature cancellation of long-running operations.

Manual Validation Scenarios: Included complete end-to-end testing scenarios that agents should run after making changes:

library(duckdb)
con <- dbConnect(duckdb())
dbExecute(con, "CREATE TABLE test (id INTEGER, name VARCHAR)")  
dbExecute(con, "INSERT INTO test VALUES (1, 'Alice'), (2, 'Bob')")
result <- dbGetQuery(con, "SELECT * FROM test ORDER BY id")
print(result)
dbDisconnect(con, shutdown=TRUE)

Repository Structure Guidance: Documented the layout of 1700+ C++ files, 40+ test files, and key directories with clear guidance on what should and shouldn't be modified.

System Dependencies: Provided exact installation commands for R, build tools, and optional formatting dependencies.

Expected Issues: Documented known formatting differences and expected test failures to avoid confusion.

Updated CLAUDE.md: Enhanced the existing operational instructions file with the same comprehensive information, making it consistent with the GitHub Copilot instructions and providing detailed guidance for developers working on the repository.

The instructions follow the firm directive requirement to "always reference these instructions first and fallback to search or bash commands only when you encounter unexpected information that does not match the info here."

Fixes #1483.

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[krlmlr]

@copilot: Update CLAUDE.md with these instructions.

[Copilot]

@copilot: Update CLAUDE.md with these instructions.

Updated CLAUDE.md with the comprehensive instructions from the GitHub Copilot instructions file. The file now includes validated commands with timing, critical timeout warnings, manual validation scenarios, repository structure guidance, and all the detailed information. Commit ed171b1.