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

---

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.

[season179]

Thanks @kiljacken, appreciate the drive-by — and you're right, .github/instructions/rtk.instructions.md is the more idiomatic home for this. It avoids touching the user's main copilot-instructions.md entirely, survives a Copilot /init regen, and uninstall becomes a single rm. Cleaner all around.

That said, I'd still like to merge this PR first as the immediate fix for #1512 — users hitting the clobber bug today need the v2 marker path, and the migration logic added here is what we'd reuse to clean up marker blocks when we move to the separate-file approach in a follow-up "v3".

One thing I'd love your input on before committing to the move: do you know how well .github/instructions/*.instructions.md is supported outside VS Code Copilot (JetBrains, Visual Studio, Copilot CLI)? If it's broadly supported, v3 is a no-brainer. If it's VS Code-only, we may want to keep copilot-instructions.md as a fallback.

Either way, happy to open a follow-up issue and update accordingly. Thanks again for the suggestion!

[xxradzip]

@season179 @kiljacken placing the instructions in .github/instructions/rtk.instructions.md does not work in Copilot CLI 1.0.37. It still can only see this on a fresh start:

Custom Instructions

3/3 enabled · Changes apply to the current session only.

User (1/1)
❯ [✓] Home copilot-instructions.md (/Users/cut_it_out_gdpr_cut_n_burn/.copilot/copilot-instructions.md)

Repository (2/2)
  [✓] .github/copilot-instructions.md (.github/copilot-instructions.md)
  [✓] AGENTS.md (AGENTS.md)

[kiljacken]

@xxradzip Might need to add markdown frontmatter with files it should apply to:

---
applyTo: "**"
---
Rest of the instruction goes here...