chore: drop Laravel Boost; make AGENTS.md the canonical agent doc

- Remove laravel/boost (+ its deps laravel/mcp, laravel/roster) and Boost's
  .cursor/ skills + MCP registration. No version sweep (composer remove).
- AGENTS.md is now the hand-maintained source of truth (was Boost-generated):
  folds in the CLAUDE.md content + a Conventions section codifying the migration
  FK rule (unsignedInteger for INT-PK refs, not foreignId — MySQL error 3780)
  and the MySQL/PostgreSQL/SQLite cross-DB requirement.
- CLAUDE.md / GEMINI.md / .github/copilot-instructions.md are now gitignored
  symlinks to AGENTS.md, created by bin/ai-docs.php (composer run ai-docs, also
  wired into post-autoload-dump).

bin/ai-docs.php

@@ -0,0 +1,31 @@
+<?php
+
+/*
+ * Creates the per-tool agent docs as symlinks to the canonical AGENTS.md.
+ * These symlinks are gitignored; this runs from composer's post-autoload-dump
+ * (and can be run manually via `composer run ai-docs`).
+ */
+
+chdir(__DIR__.'/..');
+
+$links = [
+    'CLAUDE.md' => 'AGENTS.md',
+    'GEMINI.md' => 'AGENTS.md',
+    '.github/copilot-instructions.md' => '../AGENTS.md',
+];
+
+@mkdir('.github');
+
+foreach ($links as $link => $target) {
+    if (is_link($link)) {
+        continue;
+    }
+    if (file_exists($link)) {
+        @unlink($link); // replace a stale regular file (e.g. the old committed CLAUDE.md)
+    }
+    if (@symlink($target, $link)) {
+        echo "ai-docs: linked {$link} -> {$target}\n";
+    } else {
+        fwrite(STDERR, "ai-docs: could not symlink {$link} (symlinks may require privileges on Windows)\n");
+    }
+}