docs: comprehensive copilot instructions & spec-kit setup (#5386)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/agents/speckit.git.remote.agent.md

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+---
+description: Detect Git remote URL for GitHub integration
+---
+
+
+<!-- Extension: git -->
+<!-- Config: .specify/extensions/git/ -->
+# Detect Git Remote URL
+
+Detect the Git remote URL for integration with GitHub services (e.g., issue creation).
+
+## Prerequisites
+
+- Check if Git is available by running `git rev-parse --is-inside-work-tree 2>/dev/null`
+- If Git is not available, output a warning and return empty:
+  ```
+  [specify] Warning: Git repository not detected; cannot determine remote URL
+  ```
+
+## Execution
+
+Run the following command to get the remote URL:
+
+```bash
+git config --get remote.origin.url
+```
+
+## Output
+
+Parse the remote URL and determine:
+
+1. **Repository owner**: Extract from the URL (e.g., `github` from `https://github.com/github/spec-kit.git`)
+2. **Repository name**: Extract from the URL (e.g., `spec-kit` from `https://github.com/github/spec-kit.git`)
+3. **Is GitHub**: Whether the remote points to a GitHub repository
+
+Supported URL formats:
+- HTTPS: `https://github.com/<owner>/<repo>.git`
+- SSH: `git@github.com:<owner>/<repo>.git`
+
+> [!CAUTION]
+> ONLY report a GitHub repository if the remote URL actually points to github.com.
+> Do NOT assume the remote is GitHub if the URL format doesn't match.
+
+## Graceful Degradation
+
+If Git is not installed, the directory is not a Git repository, or no remote is configured:
+- Return an empty result
+- Do NOT error — other workflows should continue without Git remote information