fix(doctor): update broken Claude plugin link (GH#623)

The bd doctor command was pointing to a non-existent README anchor
(#claude-code-plugin). Updated to point to docs/PLUGIN.md which
contains the actual plugin installation instructions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/doctor/claude.go

@@ -68,7 +68,7 @@ func CheckClaude() DoctorCheck {
 			Fix: "Set up Claude integration:\n" +
 				"  Option 1: Install the beads plugin (recommended)\n" +
 				"    • Provides hooks, slash commands, and MCP tools automatically\n" +
-				"    • See: https://github.com/steveyegge/beads#claude-code-plugin\n" +
+				"    • See: https://github.com/steveyegge/beads/blob/main/docs/PLUGIN.md\n" +
 				"\n" +
 				"  Option 2: CLI-only mode\n" +
 				"    • Run 'bd setup claude' to add SessionStart/PreCompact hooks\n" +