fix(hook): preserve existing Copilot instructions

[season179]

Summary

• Preserve existing .github/copilot-instructions.md content when installing Copilot support.
• Add an RTK instruction block with <!-- rtk-instructions v2 --> markers so future installs update idempotently.
• Migrate legacy unmarked RTK Copilot instructions without duplicating the RTK section.

Fixes #1512

Root Cause

run_copilot() used write_if_changed() for .github/copilot-instructions.md. When the file already existed with user-authored Copilot rules, any content mismatch caused RTK to atomically replace the
whole file with RTK instructions.

Test plan

• rtk cargo +1.92.0 test patch_copilot_instructions
• rtk cargo fmt --all -- --check
• rtk git diff --check
• Manual temp-project check: running init --copilot twice preserved existing Copilot rules and produced one RTK marker block

Notes

Full rtk cargo +1.92.0 test was attempted and reached the suite, but failed in unrelated existing areas: curl truncation expectations, tracking DB creation in this sandbox, and a global uninstall test
touching /Users/season/.config/opencode/plugins/rtk.ts. The new Copilot tests passed.

[CLAassistant]

All committers have signed the CLA.

[pszymkowiak]

[w] wshm · Automated triage by AI

📊 Automated PR Analysis

🐛 Type	bug-fix
🟡 Risk	medium

Summary

This PR fixes the Copilot instructions installation to preserve existing user-authored content in .github/copilot-instructions.md instead of overwriting it. It introduces HTML comment markers (<!-- rtk-instructions v2 -->) for idempotent upserts and includes migration logic to strip legacy unmarked RTK instruction blocks.

Review Checklist

• Tests present
• Breaking change
• Docs updated

Linked issues: #1512

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Analyzed automatically by wshm · This is an automated analysis, not a human review.

[kiljacken]

Drive by comment, but I think the more idiomatic way to manage this would be to create a .github/instructions/rtk.instruction.md.

Avoids messing with the users existing instructions, and reduces the risk of clobbering the RTK instructions if the user runs /init to regenerate their copilot-instructions.