chore(claude): add symlinks for Claude Code support to GitHub configs (#45204)

## Summary of the Pull Request

Adds Claude Code support by creating symbolic links under `.claude/`
that point to existing GitHub Copilot configuration files in `.github/`.
This enables Claude Code to use the same AI contributor guidance,
agents, prompts, instructions, and skills without duplicating content.

## Detailed Description of the Pull Request / Additional comments

This PR creates a `.claude/` directory with symbolic links mapping
Claude Code's expected paths to existing GitHub Copilot configurations:

| Claude Code Path | → | GitHub Copilot Source |
|------------------|---|----------------------|
| `.claude/CLAUDE.md` | → | `.github/copilot-instructions.md` |
| `.claude/agents/` | → | `.github/agents/` |
| `.claude/commands/` | → | `.github/prompts/` |
| `.claude/rules/` | → | `.github/instructions/` |
| `.claude/skills/` | → | `.github/skills/` |

**Key benefits:**
- Single source of truth — edits to `.github/` files automatically apply
to Claude Code
- Directory symlinks ensure new files (agents, prompts, skills) are
picked up without updating the mapping
- `.gitattributes` updated with `symlink` hint for `.claude/**`

**Windows users cloning this repo need:**
- Developer Mode enabled, OR admin privileges
- `git config --global core.symlinks true` before cloning

## Validation Steps Performed

- [x] Verified symlinks resolve correctly on Windows (`Get-ChildItem
.claude` shows targets)
- [x] Confirmed content is readable through symlinks (`Get-Content
.claude\CLAUDE.md`)
- [x] Verified Git indexes files as symlinks (mode `120000` in `git
ls-files -s`)
- [x] Confirmed symlink targets stored with forward slashes for
cross-platform compatibility
- [x] No automated tests required — changes are config/symlinks only
with no runtime impact

.claude/CLAUDE.md

@@ -0,0 +1 @@
+../.github/copilot-instructions.md

.claude/agents

@@ -0,0 +1 @@
+../.github/agents

.claude/commands

@@ -0,0 +1 @@
+../.github/prompts

.claude/rules

@@ -0,0 +1 @@
+../.github/instructions

.claude/skills

@@ -0,0 +1 @@
+../.github/skills